Generic SCSI target mid-level for Linux (SCST) ============================================== Version 3.3.0, XX XXXXX 2014 ---------------------------- SCST is designed to provide unified, consistent interface between SCSI target drivers and Linux kernel and simplify target drivers development as much as possible. Detail description of SCST's features and internals could be found on its Internet page http://scst.sourceforge.net. SCST supports the following I/O modes: * Pass-through mode with one to many relationship, i.e. when multiple initiators can connect to the exported pass-through devices, for the following SCSI devices types: disks (type 0), tapes (type 1), processors (type 3), CDROMs (type 5), MO disks (type 7), medium changers (type 8) and RAID controllers (type 0xC). * FILEIO mode, which allows to use files on file systems or block devices as virtual remotely available SCSI disks or CDROMs with benefits of the Linux page cache. * BLOCKIO mode, which performs direct block IO with a block device, bypassing page-cache for all operations. This mode works ideally with high-end storage HBAs and for applications that either do not need caching between application and disk or need the large block throughput. * User space mode using scst_user device handler, which allows to implement in the user space high performance virtual SCSI devices. Comparing with fully in-kernel dev handlers this mode has very low overhead (few %%). * "Performance" device handlers, which provide in pseudo pass-through mode a way for direct performance measurements without overhead of actual data transferring from/to underlying SCSI device. In addition, SCST supports advanced per-initiator access and devices visibility management, so different initiators could see different set of devices with different access permissions. See below for details. Full list of SCST features and comparison with other Linux targets you can find on http://scst.sourceforge.net/comparison.html. Installation ------------ Only vanilla kernels from kernel.org and RHEL/CentOS 5.2 kernels are supported, but SCST should work on other (vendors') kernels, if you manage to successfully compile on them. The main problem with vendors' kernels is that they often contain patches, which will appear only in the next version of the vanilla kernel, therefore it's quite hard to track such changes. Thus, if during compilation for some vendor kernel your compiler complains about redefinition of some symbol, you should either switch to vanilla kernel, or add or change as necessary the corresponding to that symbol "#if LINUX_VERSION_CODE" statement. Default sysfs interface supports only kernels 2.6.26 and higher, because in 2.6.26 internal kernel's sysfs interface had a major change, which made it heavily incompatible with pre-2.6.26 version. But with the obsolete procfs interface kernels 2.6.16+ are supported. At first, make sure that the link "/lib/modules/`you_kernel_version`/build" points to the source code for your currently running kernel. Then you should consider to apply necessary kernel patches. SCST has the following patches for the kernel in the "kernel" subdirectory. All of them are optional, so, if you don't need the corresponding functionality, you may not apply them. 1. readahead-2.6.X.patch. This patch fixes problem in Linux readahead subsystem and greatly improves performance for software RAIDs. See http://sourceforge.net/mailarchive/forum.php?thread_name=a0272b440906030714g67eabc5k8f847fb1e538cc62%40mail.gmail.com&forum_name=scst-devel thread for more details. It is included in the mainstream kernels 2.6.33 and 2.6.32.11. 2. readahead-context-2.6.X.patch. This is backported from 2.6.31 version of the context readahead patch http://lkml.org/lkml/2009/4/12/9, big thanks to Wu Fengguang. This is a performance improvement patch. It is included in the mainstream kernel 2.6.31. Then, to compile SCST type 'make scst'. It will build SCST itself and its device handlers. To install them type 'make scst_install'. The driver modules will be installed in '/lib/modules/`you_kernel_version`/extra'. In addition, scst.h, scst_debug.h as well as Module.symvers or Modules.symvers will be copied to '/usr/local/include/scst'. The first file contains all SCST's public data definition, which are used by target drivers. The other ones support debug messages logging and build process. Then you can load any module by typing 'modprobe module_name'. The names are: - scst - SCST itself - scst_disk - device handler for disks (type 0) - scst_tape - device handler for tapes (type 1) - scst_processor - device handler for processors (type 3) - scst_cdrom - device handler for CDROMs (type 5) - scst_modisk - device handler for MO disks (type 7) - scst_changer - device handler for medium changers (type 8) - scst_raid - device handler for storage array controller (e.g. raid) (type C) - scst_vdisk - device handler for virtual disks (file, device or ISO CD image). - scst_user - user space device handler Then, to see your devices remotely, you need to add a corresponding LUN for them (see below how). By default, no local devices are seen remotely. There must be LUN 0 in each LUNs set (security group), i.e. LUs numeration must not start from, e.g., 1. Otherwise you will see no devices on remote initiators and SCST core will write into the kernel log message: "tgt_dev for LUN 0 not found, command to unexisting LU?" It is highly recommended to use scstadmin utility for configuring devices and security groups. The flow of SCST initialization should be as the following: 1. Load of SCST modules with necessary module parameters, if needed. 2. Configure targets, devices, LUNs, etc. using either scstadmin (recommended), or the sysfs interface directly as described below. If you experience problems during modules load or running, check your kernel logs (or run dmesg command for the few most recent messages). IMPORTANT: Without loading appropriate device handler, corresponding devices ========= will be invisible for remote initiators, which could lead to holes in the LUN addressing, so automatic device scanning by remote SCSI mid-level could not notice the devices. Therefore you will have to add them manually via 'echo "- - -" >/sys/class/scsi_host/hostX/scan', where X - is the host number. IMPORTANT: Working of target and initiator on the same host is ========= supported, except the following 2 cases: swap over target exported device and using a writable mmap over a file from target exported device. The latter means you can't mount a file system over target exported device. In other words, you can freely use any sg, sd, st, etc. devices imported from target on the same host, but you can't mount file systems or put swap on them. This is a limitation of Linux memory/cache manager, because in this case a memory allocation deadlock is possible like: system needs some memory -> it decides to clear some cache -> the cache is needed to be written on a target exported device -> initiator sends request to the target located on the same system -> the target needs memory -> the system needs even more memory -> deadlock. IMPORTANT: In the current version simultaneous access to local SCSI devices ========= via standard high-level SCSI drivers (sd, st, sg, etc.) and SCST's target drivers is unsupported. Especially it is important for execution via sg and st commands that change the state of devices and their parameters, because that could lead to data corruption. If any such command is done, at least related device handler(s) must be restarted. For block devices READ/WRITE commands using direct disk handler are generally safe. To uninstall, type 'make scst_uninstall'. Creating a kernel patch or patched kernel ----------------------------------------- You can use generate-kernel-patch or generate-patched-kernel scripts in the scripts/ subdirectory to convert SCST source tree as it exists in the Subversion repository to a Linux kernel patch or generate a kernel source tree with the SCST patches applied correspondingly. This subdirectory exists only in the SVN tree. Example how to use generate-kernel-patch you can find at "How To install SCST on Ubutuntu 15.04 with in-tree kernel patches" https://gist.github.com/chrwei/42f8bbb687290b04b598, thanks to Chris Weiss. Script rebuild-rhel-kernel-rpm might also be useful in this regard. Migration from the obsolete proc interface ------------------------------------------ Sysfs enabled scstadmin supports old procfs config file format, so with it you should do the following steps to migrate your proc-based configuration to the sysfs interface: 1. Load SCST modules 2. Run "scstadmin -config old_config_file" 3. Run "scstadmin -write_config new_config_file" 4. Check new_config_file and make sure it has everything written properly. 5. Start using "scstadmin -config new_config_file" to configure SCST. Usage in failover mode ---------------------- It is recommended to use TEST UNIT READY ("tur") command to check if SCST target is alive in MPIO configurations. Device handlers --------------- Device specific drivers (device handlers) are plugins for SCST, which help SCST to analyze incoming requests and determine parameters, specific to various types of devices. If an appropriate device handler for a SCSI device type isn't loaded, SCST doesn't know how to handle devices of this type, so they will be invisible for remote initiators (more precisely, "LUN not supported" sense code will be returned). In addition to device handlers for real devices, there are VDISK, user space and "performance" device handlers. VDISK device handler works over files on file systems and makes from them virtual remotely available SCSI disks or CDROM's. In addition, it allows to work directly over a block device, e.g. local IDE or SCSI disk or ever disk partition, where there is no file systems overhead. Using block devices comparing to sending SCSI commands directly to SCSI mid-level via scsi_do_req()/scsi_execute_async() has advantage that data are transferred via system cache, so it is possible to fully benefit from caching and read ahead performed by Linux's VM subsystem. The only disadvantage here that in the FILEIO mode there is superfluous data copying between the cache and SCST's buffers. This issue is going to be addressed in one of the future releases. Virtual CDROM's are useful for remote installation. See below for details how to setup and use VDISK device handler. SCST user space device handler provides an interface between SCST and the user space, which allows to create pure user space devices. The simplest example, where one would want it is if he/she wants to write a VTL. With scst_user he/she can write it purely in the user space. Or one would want it if he/she needs some sophisticated for kernel space processing of the passed data, like encrypting them or making snapshots. "Performance" device handlers for disks, MO disks and tapes in their exec() method skip (pretend to execute) all READ and WRITE operations and thus provide a way for direct link performance measurements without overhead of actual data transferring from/to underlying SCSI device. NOTE: Since "perf" device handlers on READ operations don't touch the ==== commands' data buffer, it is returned to remote initiators as it was allocated, without even being zeroed. Thus, "perf" device handlers impose some security risk, so use them with caution. Compilation options ------------------- There are the following compilation options, that could be commented in/out in Makefile and scst.h: - CONFIG_SCST_DEBUG - if defined, turns on some debugging code, including some logging. Makes the driver considerably bigger and slower, producing large amount of log data. - CONFIG_SCST_TRACING - if defined, turns on ability to log events. Makes the driver considerably bigger and leads to some performance loss. - CONFIG_SCST_EXTRACHECKS - if defined, adds extra validity checks in the various places. - CONFIG_SCST_USE_EXPECTED_VALUES - if not defined (default), initiator supplied expected data transfer length and direction will be used only for verification purposes to return error or warn in case if one of them is invalid. Instead, locally decoded from SCSI command values will be used. This is necessary for security reasons, because otherwise a faulty initiator can crash target by supplying invalid value in one of those parameters. This is especially important in case of pass-through mode. If CONFIG_SCST_USE_EXPECTED_VALUES is defined, initiator supplied expected data transfer length and direction will override the locally decoded values. This might be necessary if internal SCST commands translation table doesn't contain SCSI command, which is used in your environment. You can know that if you enable "minor" trace level and have messages like "Unknown opcode XX for YY. Should you update scst_scsi_op_table?" in your kernel log and your initiator returns an error. Also report those messages in the SCST mailing list scst-devel@lists.sourceforge.net. Note, that not all SCSI transports support supplying expected values. You should try to enable this option if you have a not working with SCST pass-through device, for instance, an SATA CDROM. - CONFIG_SCST_DEBUG_TM - if defined, turns on task management functions debugging, when on LUN 6 some of the commands will be delayed for about 60 sec., so making the remote initiator send TM functions, eg ABORT TASK and TARGET RESET. Also define CONFIG_SCST_TM_DBG_GO_OFFLINE symbol in the Makefile if you want that the device eventually become completely unresponsive, or otherwise to circle around ABORTs and RESETs code. Needs CONFIG_SCST_DEBUG turned on. - CONFIG_SCST_DEBUG_SYSFS_EAGAIN - if defined, makes three out of four reads of sysfs attributes fail with -EAGAIN and also makes every sysfs write fail with -EAGAIN. This is useful to test -EAGAIN handling in user space tools like e.g. scstadmin. See also the documentation of the last_sysfs_mgmt_res sysfs attribute for more information. - CONFIG_SCST_STRICT_SERIALIZING - if defined, makes SCST send all commands to underlying SCSI device synchronously, one after one. This makes task management more reliable, with cost of some performance penalty. This is mostly actual for stateful SCSI devices like tapes, where the result of command's execution depends from device's settings defined by previous commands. Disk and RAID devices are stateless in the most cases. The current SCSI core in Linux doesn't allow to abort all commands reliably if they sent asynchronously to a stateful device. Turned off by default, turn it on if you use stateful device(s) and need as much error recovery reliability as possible. As a side effect of CONFIG_SCST_STRICT_SERIALIZING, on kernels below 2.6.30 no kernel patching is necessary for pass-through device handlers (scst_disk, etc.). - CONFIG_SCST_TEST_IO_IN_SIRQ - if defined, allows SCST to submit selected SCSI commands (TUR and READ/WRITE) from soft-IRQ context (tasklets). Enabling it will decrease amount of context switches and slightly improve performance. The goal of this option is to be able to measure overhead of the context switches. If after enabling this option you don't see under load in vmstat output on the target significant decrease of amount of context switches, then your target driver doesn't submit commands to SCST in IRQ context. For instance, iSCSI-SCST doesn't do that, but qla2x00t with CONFIG_QLA_TGT_DEBUG_WORK_IN_THREAD disabled - does. This option is designed to be used with vdisk NULLIO backend. WARNING! Using this option enabled with other backend than vdisk NULLIO is unsafe and can lead you to a kernel crash! - CONFIG_SCST_STRICT_SECURITY - if defined, makes SCST zero allocated data buffers. Undefining it (default) considerably improves performance and eases CPU load, but could create a security hole (information leakage), so enable it, if you have strict security requirements. - CONFIG_SCST_ABORT_CONSIDER_FINISHED_TASKS_AS_NOT_EXISTING - if defined, in case when TASK MANAGEMENT function ABORT TASK is trying to abort a command, which has already finished, remote initiator, which sent the ABORT TASK request, will receive TASK NOT EXIST (or ABORT FAILED) response for the ABORT TASK request. This is more logical response, since, because the command finished, attempt to abort it failed, but some initiators, particularly VMware iSCSI initiator, consider TASK NOT EXIST response as if the target got crazy and try to RESET it. Then sometimes get crazy itself. So, this option is disabled by default. - CONFIG_SCST_MEASURE_LATENCY - if defined, provides in "latency" files global and per-LUN average commands processing latency statistic. You can clear already measured results by writing 0 in each file. Note, you need a non-preemptible kernel to have correct results. - CONFIG_SCST_DIF_INJECT_CORRUPTED_TAGS - if defined, allows injection of corrupted DIF tags according to the Oracle specification. This functionality is working only if dif_mode doesn't contain dev_store and dif_type is 1. - CONFIG_SCST_FORWARD_MODE_PASS_THROUGH - if defined, the pass-through subsystem starts working in the forwarding mode, where reservation commands processed locally and not passed to the backend SCSI device, while COMPARE AND WRITE, EXTENDED COPY and RECEIVE COPY RESULTS commands, which normally processed locally by the SCST core, not processed locally, but passed to the backend device. Intended to be used to implement NON-OPTIMIZED ALUA state together with "forwarding" target attribute on the remote node. Disabled by default for safety. HIGHMEM kernel configurations are fully supported, but not recommended for performance reasons, except for scst_user, where they are not supported, because this module deals with user supplied memory on a zero-copy manner. If you need to use HIGHMEM enabled, consider change VMSPLIT option or use 64-bit system configuration instead. For changing VMSPLIT option (CONFIG_VMSPLIT to be precise) you should in "make menuconfig" command set the following variables: - General setup->Configure standard kernel features (for small systems): ON - General setup->Prompt for development and/or incomplete code/drivers: ON - Processor type and features->High Memory Support: OFF - Processor type and features->Memory split: according to amount of memory you have. If it is less than 800MB, you may not touch this option at all. Module parameters ----------------- Module scst supports the following parameters: - scst_threads - allows to set count of SCST's threads. By default it is CPU count. - scst_max_cmd_mem - sets maximum amount of memory in MB allowed to be consumed by the SCST commands for data buffers at any given time. By default it is approximately TotalMem/4. - scst_max_dev_cmd_mem - sets maximum amount of memory in MB allowed to be consumed by all SCSI commands of a device at any given time. By default, it is approximately 2/5 of scst_max_cmd_mem. SCST sysfs interface -------------------- Starting from 2.0.0 SCST has sysfs interface. It supports only kernels 2.6.26 and higher, because in 2.6.26 internal kernel's sysfs interface had a major change, which made it heavily incompatible with pre-2.6.26 version. If you need pre-2.6.26 kernel, you need to use obsolete procfs interface (see below). SCST sysfs interface designed to be self descriptive and self containing. This means that a high level management tool for it can be written once and automatically support any future sysfs interface changes (attributes additions or removals, new target drivers and dev handlers, etc.) without any modifications. Scstadmin is an example of such management tool. To implement that an management tool should not be implemented around drivers and their attributes, but around common rules those drivers and attributes follow. You can find those rules in SysfsRules file. For instance, each SCST sysfs file (attribute) can contain in the last line mark "[key]". It is automatically added to allow scstadmin and other management tools to see which attributes it should save in the config file. If you are doing manual attributes manipulations, you can ignore this mark. Root of SCST sysfs interface is /sys/kernel/scst_tgt. It has the following entries: - devices - this is a root subdirectory for all SCST devices - handlers - this is a root subdirectory for all SCST dev handlers - max_tasklet_cmd - specifies how many commands at max can be queued in the SCST core simultaneously on a single CPU from all connected initiators to allow processing commands on this CPU in soft-IRQ context in tasklets. If the count of the commands exceeds this value, then all of them will be processed only in SCST threads. This is to to prevent possible under heavy load starvation of processes on the CPUs serving soft IRQs and in some cases to improve performance by more evenly spreading load over available CPUs. - sgv - this is a root subdirectory for all SCST SGV caches - targets - this is a root subdirectory for all SCST targets - setup_id - allows to read and write SCST setup ID. This ID can be used in cases, when the same SCST configuration should be installed on several targets, but exported from those targets devices should have different IDs and SNs. For instance, VDISK dev handler uses this ID to generate T10 vendor specific identifier and SN of the devices. - poll_us - if polling is desired, sets how many us each SCST thread is polling its queue after it became empty in a hope that a new command can come. In some cases, polling can significantly increase IOPS, especially if low power states on CPU not disabled, because on high IOPS polling could be cheaper comparing to spending significant time on entering, then exiting CPU low power states + corresponding context switches. Disabled, i.e. set to 0, by default. - suspend - globally suspends or releases all SCSI activities on all devices. Useful for mass management, like adding or deleting LUNs. Writing to it value v: * v > 0 - suspends activities, but waits no more, than v seconds * v = 0 - suspends activities, waits indefinitely * V < 0 - releases activities. Reading from this attribute returns number of previous suspend requests. - threads - allows to read and set number of global SCST I/O threads. Those threads used with async. dev handlers, for instance, vdisk BLOCKIO or NULLIO. - trace_cmds - shows current SCST commands up to size of the sysfs buffer (4KB) - trace_mcmds - shows current SCST management commands up to size of the sysfs buffer (4KB) - trace_level - allows to enable and disable various tracing facilities. See content of this file for help how to use it. See also section "Dealing with massive logs" for more info how to make correct logs when you enabled trace levels producing a lot of logs data. - version - read-only attribute, which allows to see version of SCST and enabled optional features. - last_sysfs_mgmt_res - read-only attribute returning completion status of the last management command. In the sysfs implementation there are some problems between internal sysfs and internal SCST locking. To avoid them in some cases sysfs calls can return error with errno EAGAIN. This doesn't mean the operation failed. It only means that the operation queued and not yet completed. To wait for it to complete, an management tool should poll this file. If the operation hasn't yet completed, it will also return EAGAIN. But after it's completed, it will return the result of this operation (0 for success or -errno for error). The following two shell functions show how to do this: # Read the SCST sysfs attribute $1. See also scst/README for more information. scst_sysfs_read() { local EAGAIN val EAGAIN="Resource temporarily unavailable" while true; do if val="$(LC_ALL=C cat "$1" 2>&1)"; then echo -n "${val%\[key\]}" return 0 elif [ "${val/*: }" != "$EAGAIN" ]; then return 1 fi sleep 1 done } # Write $1 into the SCST sysfs attribute $2. See also scst/README for more # information. scst_sysfs_write() { local EAGAIN status EAGAIN="Resource temporarily unavailable" if status="$(LC_ALL=C; (echo -n "$1" > "$2") 2>&1)"; then return 0 elif [ "${status/*: }" != "$EAGAIN" ]; then return 1 fi scst_sysfs_read /sys/kernel/scst_tgt/last_sysfs_mgmt_res >/dev/null } "Devices" subdirectory contains subdirectories for each SCST devices. Content of each device's subdirectory is dev handler specific. See documentation for your dev handlers for more info about it as well as SysfsRules file for more info about common to all dev handlers rules. SCST dev handlers can have the following common entries: - block - allows to temporary block and unblock this device. See below. - exported - subdirectory containing links to all LUNs where this device was exported. - handler - if dev handler determined for this device, this link points to it. The handler can be not set for pass-through devices. - threads_num - shows and allows to set number of threads in this device's threads pool. If 0 - no threads will be created, and global SCST threads pool will be used. If <0 - creation of the threads pool is prohibited. - threads_pool_type - shows and allows to sets threads pool type. Possible values: "per_initiator" and "shared". When the value is "per_initiator" (default), each session from each initiator will use separate dedicated pool of threads. When the value is "shared", all sessions from all initiators will share the same per-device pool of threads. Valid only if threads_num attribute >0. - dump_prs - allows to dump persistent reservations information in the kernel log. - type - SCSI type of this device Attribute "block" allows to temporary block and unblock this device. "Blocking" means that no new commands for this device will go into the execution stage, but instead will be suspended just before it. The blocked state is not reached until queue of the corresponding device is completely drained. You can also call this state "frozen". It is useful in many cases, like consistent snapshots and graceful shutdown. On write "block" entry allows the following 3 types of parameters: - 1 - block device synchronously, i.e. don't return until this device becomes blocked, i.e. until queue of it is not completely drained. Can be called as many times as needed. - 11 params - block device asynchronously, i.e. return immediately. Notification about completing is delivered using SCST_EVENT_EXT_BLOCKING_DONE event. "Params" delivered to it as is in "data" payload. Can be called as many times as needed. Alternatively, status of blocking could be polled by reading this attributes until the second number reaches 0 (see below). - 0 - unblock this device. Reading from "block" entry returns two numbers separated by space: 1. How many times this device was blocked, i.e. how many times writing "0" to it is needed to unblock this device. 2. Boolean (0 or 1) if blocking, if any, is done (0) or still pending (1). See below for more information about other entries of this subdirectory of the standard SCST dev handlers. "Handlers" subdirectory contains subdirectories for each SCST dev handler. Content of each handler's subdirectory is dev handler specific. See documentation for your dev handlers for more info about it as well as SysfsRules file for more info about common to all dev handlers rules. SCST dev handlers can have the following common entries: - mgmt - this entry allows to create virtual devices and their attributes (for virtual devices dev handlers) or assign/unassign real SCSI devices to/from this dev handler (for pass-through dev handlers). - trace_level - allows to enable and disable various tracing facilities. See content of this file for help how to use it. See also section "Dealing with massive logs" for more info how to make correct logs when you enabled trace levels producing a lot of logs data. - type - SCSI type of devices served by this dev handler. See below for more information about other entries of this subdirectory of the standard SCST dev handlers. "Sgv" subdirectory contains statistic information of SCST SGV caches. It has the following entries: - None, one or more subdirectories for each existing SGV cache. - global_stats - file containing global SGV caches statistics. Each SGV cache's subdirectory has the following item: - stats - file containing statistics for this SGV caches. "Targets" subdirectory contains subdirectories for each SCST target. Content of each target's subdirectory is target specific. See documentation for your target for more info about it as well as SysfsRules file for more info about common to all targets rules. Every target should have at least the following entries: - ini_groups - subdirectory, which contains and allows to define initiator-oriented access control information, see below. - luns - subdirectory, which contains list of available LUNs in the target-oriented access control and allows to define it, see below. - sessions - subdirectory containing connected to this target sessions. - comment - this attribute can be used to store any human readable info to help identify target. For instance, to help identify the target's mapping to the corresponding hardware port. It isn't anyhow used by SCST. - enabled - using this attribute you can enable or disable this target. It allows to finish configuring it before it starts accepting new connections. 0 by default. - addr_method - used LUNs addressing method. Possible values: "Peripheral", "Flat" or "LUN". Most initiators work well with Peripheral addressing method (default), but some (HP-UX, for instance) may require the Flat method or the LUN method (e.g. IBM systems). This attribute is also available in the initiators security groups, so you can assign the addressing method on per-initiator basis. See also the "Logical unit addressing (LUN)" section in SAM-5 for more information. - black_hole - if set, all LUNs in the corresponding initiator group, default target group in this case, start "swallowing" requests from initiators. Possible values are: * 0 - disable black hole mode * 1 - immediately abort all coming SCSI commands, i.e. all SCSI commands are dropped and TM requests return that they completed. It is supposed to simulate lost front end responses. * 2 - immediately abort all coming SCSI commands and drop all coming TM commands. It is supposed to simulate logical target hang, when the target stops responding, but on the HW/TCP connection level still appears to be online. * 3 - immediately abort all coming data transfer SCSI commands, i.e. only data transfer SCSI commands are dropped, while commands like INQUIRY and TEST UNIT READY pass well. It is supposed to simulate flaky front end connectivity, when responses for small commands pass well, but big data transfers fail. * 4 - immediately abort all coming data transfer SCSI commands and drop all coming TM commands. It is supposed to simulate really flaky front end connectivity, when TM requests or responses are also lost. Modes 3 and 4 are the most evil ones, because they are not too well handled by many initiator OS'es, including Linux, so they may never recover from it. Note, dropping TM commands, i.e. not sending response on them, implemented not for all target drivers. If it's implemented for your particular target driver or not, you can find out by checking traces or the target driver's source code. - dif_capabilities - if this target supports T10-PI, returns which exact DIF capabilities this target supports. - dif_checks_failed - if this target supports T10-PI, returns statistics how many DIF errors have been detected on the corresponding processing stages on this target. It returns 3 rows of numbers with 3 numbers in each row: for target driver stage, for SCST stage and for dev handler stage. Numbers in each row: how many errors detected checking application, reference and guard tags correspondingly. Writing to this attribute resets the numbers. - cpu_mask - defines CPU affinity mask for threads serving this target. For threads serving LUNs it is used only for devices with threads_pool_type "per_initiator". - io_grouping_type - defines how I/O from sessions to this target are grouped together. This I/O grouping is very important for performance. By setting this attribute in a right value, you can considerably increase performance of your setup. This grouping is performed only if you use CFQ I/O scheduler on the target and for devices with threads_num >= 0 and, if threads_num > 0, with threads_pool_type "per_initiator". Possible values: "this_group_only", "never", "auto", or I/O group number >0. When the value is "this_group_only" all I/O from all sessions in this target will be grouped together. When the value is "never", I/O from different sessions will not be grouped together, i.e. all sessions in this target will have separate dedicated I/O groups. When the value is "auto" (default), all I/O from initiators with the same name (iSCSI initiator name, for instance) in all targets will be grouped together with a separate dedicated I/O group for each initiator name. For iSCSI this mode works well, but other transports usually use different initiator names for different sessions, so using such transports in MPIO configurations you should either use value "this_group_only", or an explicit I/O group number. This attribute is also available in the initiators security groups, so you can assign the I/O grouping on per-initiator basis. See below for more info how to use this attribute. - rel_tgt_id - allows to read or write SCSI Relative Target Port Identifier attribute. This identifier is used to identify SCSI Target Ports by some SCSI commands, mainly by Persistent Reservations commands. This identifier must be unique among all SCST targets, but for convenience SCST allows disabled targets to have not unique rel_tgt_id. In this case SCST will not allow to enable this target until rel_tgt_id becomes unique. This attribute initialized unique by SCST by default. - forwarding - if set this target is forwarding target, i.e. does not check any local SCSI events (reservations, etc.). Those event supposed to be checked on the another, requester's side. - *count*, e.g. read_io_count_kb, - statistics about executed commands and transferred data. Those attributes have speaking names built from parts: 1. Data transfer direction 2. Alignment type: not specified or unaligned (on 4K boundaries) 3. Type: IO (commands) count or amount of transferred data 4. For transferred data: measurement units For instance, read_unaligned_cmd_count means number of 4K unaligned IOs. A target driver may have also the following entries: - "hw_target" - if the target driver supports both hardware and virtual targets (for instance, an FC adapter supporting NPIV, which has hardware targets for its physical ports as well as virtual NPIV targets), this read only attribute for all hardware targets will exist and contain value 1. Subdirectory "sessions" contains one subdirectory for each connected session with name equal to name of the connected initiator with the following entries: - initiator_name - contains initiator name - force_close - optional write-only attribute, which allows to force close this session. - active_commands - contains number of active, i.e. not yet or being executed, SCSI commands in this session. - commands - contains overall number of SCSI commands in this session. - dif_checks_failed - if target of this session supports T10-PI, returns statistics how many DIF errors have been detected on the corresponding processing stages on all DIF-enabled LUNs in this session. It returns 3 rows of numbers with 3 numbers in each row: for target driver stage, for SCST stage and for dev handler stage. Numbers in each row: how many errors detected checking application, reference and guard tags correspondingly. Writing to this attribute resets the numbers. Similar statistics returned in attribute with the same name for each LUN in this session in this LUN's subdirectory, if its device configured with dif_type > 0. - read_cmd_count - number of READ SCSI commands received since beginning or last reset (writing 0 in this attribute) - read_io_count_kb - amount of data in KB read by the initiator since beginning or last reset (writing 0 in this attribute) - write_cmd_count - number of WRITE SCSI commands received since beginning or last reset (writing 0 in this attribute) - write_io_count_kb - amount of data in KB written by the initiator since beginning or last reset (writing 0 in this attribute) - bidi_cmd_count - number of BIDI SCSI commands received since beginning or last reset (writing 0 in this attribute) - bidi_io_count_kb - amount of data in KB transferred by the initiator since beginning or last reset (writing 0 in this attribute) - none_cmd_count - number of not transferring data SCSI commands (e.g. INQUIRY or TEST UNIT READY) received since beginning or last reset (writing 0 in this attribute) - unknown_cmd_count - number of unknown SCSI commands received since beginning or last reset (writing 0 in this attribute) - latency - if CONFIG_SCST_MEASURE_LATENCY enabled, contains latency statistics for this session. - *count*, e.g. read_io_count_kb, - statistics about executed commands and transferred data. See above for more details. - luns - a link pointing out to the corresponding LUNs set (security group) where this session was attached to. - One or more "lunX" subdirectories, where 'X' is a number, for each LUN this session has (see below). - other target driver specific attributes and subdirectories. See below description of the VDISK's sysfs interface for samples. Each sessions//lun subdirectory contains the following entries: - active_commands - contains number of active, i.e. not yet or being executed, SCSI commands for lun in session . - thread_pid - contains a single line with all the process identifiers (PIDs) of the kernel threads that process SCSI commands intended for lun in session . Access and devices visibility management (LUN masking) ------------------------------------------------------ Access and devices visibility management allows for an initiator or group of initiators to see different devices with different LUNs with necessary access permissions. SCST supports two modes of access control: 1. Target-oriented. In this mode you define for each target a default set of LUNs, which are accessible to all initiators, connected to that target. This is a regular access control mode, which people usually mean thinking about access control in general. For instance, in IET this is the only supported mode. 2. Initiator-oriented. In this mode you define which LUNs are accessible for each initiator. In this mode you should create for each set of one or more initiators, which should access to the same set of devices with the same LUNs, a separate security group, then add to it devices and names of allowed initiator(s). Both modes can be used simultaneously. In this case the initiator-oriented mode has higher priority, than the target-oriented, i.e. initiators are at first searched in all defined security groups for this target and, if none matches, the default target's set of LUNs is used. This set of LUNs might be empty, then the initiator will not see any LUNs from the target. You can at any time find out which set of LUNs each session is assigned to by looking where link /sys/kernel/scst_tgt/targets/target_driver/target_name/sessions/initiator_name/luns points to. To configure the target-oriented access control SCST provides the following interface. Each target's sysfs subdirectory (/sys/kernel/scst_tgt/targets/target_driver/target_name) has "luns" subdirectory. This subdirectory contains the list of already defined target-oriented access control LUNs for this target as well as file "mgmt". This file has the following commands, which you can send to it, for instance, using "echo" shell command. You can always get a small help about supported commands by looking inside this file. "Parameters" are one or more param_name=value pairs separated by ';'. - "add H:C:I:L lun [parameters]" - adds a pass-through device with host:channel:id:lun with LUN "lun". Optionally, the device could be marked as read only by using parameter "read_only". The recommended way to find out H:C:I:L numbers is use of lsscsi utility. - "replace H:C:I:L lun [parameters]" - replaces by pass-through device with host:channel:id:lun existing with LUN "lun" device with generation of INQUIRY DATA HAS CHANGED Unit Attention. If the old device doesn't exist, this command acts as the "add" command. Optionally, the device could be marked as read only by using parameter "read_only". The recommended way to find out H:C:I:L numbers is use of lsscsi utility. - "add VNAME lun [parameters]" - adds a virtual device with name VNAME with LUN "lun". Optionally, the device could be marked as read only by using parameter "read_only". - "replace VNAME lun [parameters]" - replaces by virtual device with name VNAME existing with LUN "lun" device with generation of INQUIRY DATA HAS CHANGED Unit Attention. If the old device doesn't exist, this command acts as the "add" command. Optionally, the device could be marked as read only by using parameter "read_only". - "del lun" - deletes LUN lun - "clear" - clears the list of devices To configure the initiator-oriented access control SCST provides the following interface. Each target's sysfs subdirectory (/sys/kernel/scst_tgt/targets/target_driver/target_name) has "ini_groups" subdirectory. This subdirectory contains the list of already defined security groups for this target as well as file "mgmt". This file has the following commands, which you can send to it, for instance, using "echo" shell command. You can always get a small help about supported commands by looking inside this file. - "create GROUP_NAME" - creates a new security group. - "del GROUP_NAME" - deletes a new security group. Each security group's subdirectory contains 2 subdirectories: initiators and luns as well as the following attributes: addr_method, cpu_mask and io_grouping_type, black_hole. See above description of them. Each "initiators" subdirectory contains list of added to this groups initiator as well as as well as file "mgmt". This file has the following commands, which you can send to it, for instance, using "echo" shell command. You can always get a small help about supported commands by looking inside this file. - "add INITIATOR_NAME" - adds initiator with name INITIATOR_NAME to the group. - "del INITIATOR_NAME" - deletes initiator with name INITIATOR_NAME from the group. - "move INITIATOR_NAME DEST_GROUP_NAME" moves initiator with name INITIATOR_NAME from the current group to group with name DEST_GROUP_NAME. - "clear" - deletes all initiators from this group. For "add" and "del" commands INITIATOR_NAME can be a simple DOS-type patterns, containing '*' and '?' symbols. '*' means match all any symbols, '?' means match only any single symbol. For instance, "blah.xxx" will match "bl?h.*". Additionally, you can use negative sign '!' to revert the value of the pattern. For instance, "ah.xxx" will match "!bl?h.*". Each "luns" subdirectory contains the list of already defined LUNs for this group as well as file "mgmt". Content of this file as well as list of available in it commands is fully identical to the "luns" subdirectory of the target-oriented access control. Examples: - echo "create INI" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/mgmt - creates security group INI for target iqn.2006-10.net.vlnb:tgt1. - echo "add 2:0:1:0 11" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI/luns/mgmt - adds a pass-through device sitting on host 2, channel 0, ID 1, LUN 0 to group with name INI as LUN 11. - echo "add disk1 0" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI/luns/mgmt - adds a virtual disk with name disk1 to group with name INI as LUN 0. - echo "add 21:*:e0:?b:83:*" >/sys/kernel/scst_tgt/targets/21:00:00:a0:8c:54:52:12/ini_groups/INI/initiators/mgmt - adds a pattern to group with name INI to Fibre Channel target with WWN 21:00:00:a0:8c:54:52:12, which matches WWNs of Fibre Channel initiator ports. Consider you need to have an iSCSI target with name "iqn.2007-05.com.example:storage.disk1.sys1.xyz", which should export virtual device "dev1" with LUN 0 and virtual device "dev2" with LUN 1, but initiator with name "iqn.2007-05.com.example:storage.disk1.spec_ini.xyz" should see only virtual device "dev2" read only with LUN 0. To achieve that you should do the following commands: # echo "iqn.2007-05.com.example:storage.disk1.sys1.xyz" >/sys/kernel/scst_tgt/targets/iscsi/mgmt # echo "add dev1 0" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/luns/mgmt # echo "add dev2 1" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/luns/mgmt # echo "create SPEC_INI" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/ini_groups/mgmt # echo "add dev2 0 read_only=1" \ >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/ini_groups/SPEC_INI/luns/mgmt # echo "iqn.2007-05.com.example:storage.disk1.spec_ini.xyz" \ >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/ini_groups/SPEC_INI/initiators/mgmt For Fibre Channel or SAS in the above example you should use target's and initiator ports WWNs instead of iSCSI names. It is highly recommended to use scstadmin utility instead of described in this section low level interface. IMPORTANT ========= There must be LUN 0 in each set of LUNs, i.e. LUs numeration must not start from, e.g., 1. Otherwise you will see no devices on remote initiators and SCST core will write into the kernel log message: "tgt_dev for LUN 0 not found, command to unexisting LU?" IMPORTANT ========= All the access control must be fully configured BEFORE the corresponding target is enabled. When you enable a target, it will immediately start accepting new connections, hence creating new sessions, and those new sessions will be assigned to security groups according to the *currently* configured access control settings. For instance, to the default target's set of LUNs, instead of "HOST004" group as you may need, because "HOST004" doesn't exist yet. So, you must configure all the security groups before new connections from the initiators are created, i.e. before the target enabled. VDISK device handler -------------------- Starting from 2.0.0 VDISK device handler uses sysfs interface. The procfs interface is obsolete and will be removed in one of the next versions. VDISK has 4 built-in dev handlers: vdisk_fileio, vdisk_blockio, vdisk_nullio and vcdrom. Roots of their sysfs interface are /sys/kernel/scst_tgt/handlers/handler_name, e.g. for vdisk_fileio: /sys/kernel/scst_tgt/handlers/vdisk_fileio. Each root has the following entries: - None, one or more links to devices with name equal to names of the corresponding devices. - trace_level - allows to enable and disable various tracing facilities. See content of this file for help how to use it. See also section "Dealing with massive logs" for more info how to make correct logs when you enabled trace levels producing a lot of logs data. - mgmt - main management entry, which allows to add/delete VDISK devices with the corresponding type. The "mgmt" file has the following commands, which you can send to it, for instance, using "echo" shell command. You can always get a small help about supported commands by looking inside this file. "Parameters" are one or more param_name=value pairs separated by ';'. - echo "add_device device_name [parameters]" - adds a virtual device with name device_name and specified parameters (see below) - echo "del_device device_name" - deletes a virtual device with name device_name. Handler vdisk_fileio provides FILEIO mode to create virtual devices. This mode uses as backend files and accesses to them using regular read()/write() file calls. This allows to use full power of Linux page cache. The following parameters possible for vdisk_fileio: - filename - specifies path and file name of the backend file. The path must be absolute. - blocksize - specifies block size used by this virtual device. The block size must be power of 2 and >= 512 bytes. Default is 512. - write_through - disables write back caching. Note, this option has sense only if you also *manually* disable write-back cache in *all* your backstorage devices and make sure it's actually disabled, since many devices are known to lie about this mode to get better benchmark results. Default is 0. - read_only - read only. Default is 0. - o_direct - disables both read and write caching. This mode isn't currently fully implemented, you should use user space fileio_tgt program in O_DIRECT mode instead (see below). - nv_cache - enables "non-volatile cache" mode. In this mode it is assumed that the target has a GOOD UPS with ability to cleanly shutdown target in case of power failure and it is software/hardware bugs free, i.e. all data from the target's cache are guaranteed sooner or later to go to the media. Hence all data synchronization with media operations, like SYNCHRONIZE_CACHE, are ignored in order to bring more performance. Also in this mode target reports to initiators that the corresponding device has write-through cache to disable all write-back cache workarounds used by initiators. Use with extreme caution, since in this mode after a crash of the target journaled file systems don't guarantee the consistency after journal recovery, therefore manual fsck MUST be ran. Note, that since usually the journal barrier protection (see "IMPORTANT" note below) turned off, enabling NV_CACHE could change nothing from data protection point of view, since no data synchronization with media operations will go from the initiator. This option overrides "write_through" option. Disabled by default. - thin_provisioned - enables thin provisioning facility, when remote initiators can unmap blocks of storage, if they don't need them anymore. Backend storage also must support this facility. - tst - allows to specify TST control mode page field. It specifies the type of task set in the device. Possible values are: 0 - the device maintains one task set for all I_T nexuses and 1 - the device maintains separate task sets for each I_T nexus. Default - 1. - removable - with this flag set the device is reported to remote initiators as removable. - rotational - if set, this device reported as rotational. Otherwise, it is reported as non-rotational (SSD, etc.) - zero_copy - if set, then this device uses zero copy access to the page cache. At the moment, only read side zero copy is implemented. - dif_mode - specifies which T10-PI, or DIF, mode this device will use. See SCSI standards from more info about T10-PI. Available DIF modes (can be combined using '|'): * tgt - DIF tags are checked on the target hardware, if supported * scst - DIF tags are checked inside SCST core * dev_check - DIF tags are checked inside backend device. No DIF tags storing is required, but optionally possible. * dev_store - DIF tags are stored inside backend device on the WRITE path and read from it on the READ path. No DIF tags checking is required, but optionally possible. For instance, if only tgt DIF mode specified, then target driver, serving this device, will inside hardware check, then STRIP DIF tags from SCSI commands on the WRITE path and generate, then INSERT DIF tags into SCSI commands on the READ path, so neither SCST core, nor dev handler will see them. Similarly, if only scst DIF mode specified, then target driver will PASS DIF tags into SCST core, which then check/STRIP/generate/INSERT them, so dev handler will not see them. If only dev_check DIF mode specified, then both target driver and SCST core will PASS DIF tags into the dev handler, which is then responsible to check them in the backend hardware. If only dev_store specified, then DIF tags will only be stored by the dev handler in the backend hardware without checking at any level. If all "tgt|scst|dev_check|dev_store" DIF mode specified, then all target driver, SCST core and dev handler will check DIF tags, then dev handler will store them in the backend hardware. - dif_type - specifies which DIF SCSI type this device will use. - dif_static_app_tag - specifies fixed (static) DIF application tag for this device. - dif_filename - specifies full path to filename, where DIF tags will be stored. Handler vdisk_blockio provides BLOCKIO mode to create virtual devices. This mode performs direct block I/O with a block device, bypassing the page cache for all operations. This mode works ideally with high-end storage HBAs and for applications that either do not need caching between application and disk or need the large block throughput. See below for more info. The following parameters possible for vdisk_blockio: filename, blocksize, nv_cache, read_only, removable, rotational, thin_provisioned, tst, dif_mode, dif_type, dif_static_app_tag, dif_filename. See vdisk_fileio above for description of those parameters. Handler vdisk_nullio provides NULLIO mode to create virtual devices. In this mode no real I/O is done, but success returned to initiators. Intended to be used for performance measurements at the same way as "*_perf" handlers. The following parameters possible for vdisk_nullio: blocksize, read_only, removable, tst. See vdisk_fileio above for description of those parameters. vdisk_nullio devices have the following two additional attributes: - dummy - if this flag is set, LUNs corresponding to this device will not appear at the initiator side. This is because SCST will set the PERIPHERAL QUALIFIER qualifier field to 1 (not connected) and the PERIPHERAL DEVICE TYPE to 0x1f (no device) in the INQUIRY response. See also SPC-4 for more information. It is designed to be used as a "dummy" placeholder on LUN 0, if LUN 0 is not desired. - read_zero - if this flag is set, reading from a vdisk_nullio device returns a buffer filled with byte 0x00. If this flag is cleared (which is the default behavior), the buffer returned to the initiator is not cleared. Although this results in slightly faster operation this is a security hole since any data that is present in kernel memory can be returned to the initiator. Handler vcdrom allows emulation of a virtual CDROM device using an ISO file as backend. It has only single parameter: tst. For example: echo "add_device disk1 filename=/disk1; blocksize=4096; nv_cache=1" >/sys/kernel/scst_tgt/handlers/vdisk_fileio/mgmt will create a FILEIO virtual device disk1 with backend file /disk1 with block size 4K and NV_CACHE enabled. Each vdisk_fileio's device has the following attributes in /sys/kernel/scst_tgt/devices/device_name: - filename - contains path and file name of the backend file. - blocksize - contains block size used by this virtual device. - write_through - contains status of write back caching of this virtual device. - sync - writing into this attribute causes the page cache contents to be flushed to disk. - read_only - contains read only status of this virtual device. - o_direct - contains O_DIRECT status of this virtual device. - inq_vend_specific - Vendor specific data that will be reported via either bytes 36..55 or bytes 96..256 of the INQUIRY response, depending on whether this field is <= 20 or > 20 bytes long. - nv_cache - contains NV_CACHE status of this virtual device. - prod_id - PRODUCT IDENTIFICATION as reported via the INQUIRY response. The default value for this field is the SCST device name. - prod_rev_lvl - PRODUCT REVISION LEVEL as reported via the INQUIRY response. The default value for this field is " 300". - scsi_device_name - optional SCSI target device name to which this SCST device belongs to (in SCSI terminology all SCST devices called Logical Units). See SPC for more info. - tst - contains TST field of SCSI Control mode page. See SPC-4 for more details about this field. - thin_provisioned - contains thin provisioning status of this virtual device. - gen_tp_soft_threshold_reached_UA - for thin provisioned devices writing of anything into this write-only attribute will generate THIN PROVISIONING SOFT THRESHOLD REACHED Unit Attention to all connected to this device initiators. - removable - contains removable status of this virtual device. - rotational - contains rotational status of this virtual device. - size_mb - contains size of this virtual device in MB. - pr_file_name - Full path of the file or block device in which to store persistent reservation information. The default value for this attribute is /var/lib/scst/pr/${device_name}. Writing a new value into this sysfs attribute is only allowed if the device is not exported. Modifying this sysfs attribute causes the persistent reservation state to be reloaded. - t10_dev_id - contains and allows to set T10 vendor specific identifier for Device Identification VPD page (0x83) of INQUIRY data. By default VDISK handler always generates t10_dev_id for every new created device at creation time based on the device name and scst_vdisk_ID scst_vdisk.ko module parameter for procfs (see below) or the SCST setup_id when using the sysfs interface (see above). Note: some initiators, e.g. VMware's ESXi or MS Hyper-V, only looks at the first eight characters of t10_dev_id. You have to make sure that these first eight characters are unique or VMware will consider these devices as identical. - eui64_id - allows to set the EUI-64 based device identifier in the SCSI device identification VPD page (83h). This identifier must be 8, 12 or 16 bytes long and must be specified in hexadecimal format (EUI = Extended Unique Identifier). A leading "0x" is allowed but is not required. Writing a newline into this attribute discards the EUI-64 identifier. If neither eui64_id nor naa_id have been set the first eight bytes of the t10_dev_id are used as the EUI-64 ID. If naa_id has been set but eui64_id has not been set no EUI-64 identifier is reported in the SCSI device identification VPD page. If eui64_id has been set the value of this attribute is reported as the EUI-64 ID. The first three bytes of an EUI-64 ID are a so-called organizationally unique identifier (OUI). The remaining bytes may be chosen by the organization that owns the OUI. For more information about OUIs, see also http://standards.ieee.org/develop/regauth/oui/public.html. - naa_id - allows to set the NAA ID in the SCSI INQUIRY response (NAA = Network Address Authority). This identifier must be 8 or 16 bytes long and must be specified in hex format. A leading "0x" is allowed but is not required. Writing a newline into this attribute discards the NAA ID. If this ID is set it is reported in the SCSI VPD device identification page (83h). More information about NAA identifiers can be found in the following documents: * ANSI T11 committee, Fibre Channel Framing and Signaling Interface - 4 (FC-FS-4) rev 0.50, May 2014 (http://www.t11.org/). * IETF, RFC 3980 - T11 Network Address Authority (NAA) Naming Format for iSCSI Node Names, February 2005 (https://tools.ietf.org/html/rfc3980). - t10_vend_id - Contents of the T10 VENDOR IDENTIFICATION field of the INQUIRY response. The default value for this field is "SCST_BIO" for vdisk_block devices and "SCST_FIO" for vdisk_fileio devices. - usn - contains the virtual device's serial number of INQUIRY data. It is created at the device creation time based on the device name and scst_vdisk_ID scst_vdisk.ko module parameter for procfs (see below) or the SCST setup_id when using the sysfs interface (see above). - type - contains SCSI type of this virtual device. - resync_size - write only attribute, which makes vdisk_fileio to rescan size of the backend file. It is useful if you changed it, for instance, if you resized it. - vend_specific_id - Vendor specific ID as reported via the Device Identification VPD page (83h). The default value for this attribute is the value of the t10_dev_id attribute. For example: /sys/kernel/scst_tgt/devices/disk1 |-- block |-- blocksize |-- exported | |-- export0 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt/luns/0 | |-- export1 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt/ini_groups/INI/luns/0 | |-- export2 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt1/luns/0 | |-- export3 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI1/luns/0 | |-- export4 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI2/luns/0 |-- filename |-- handler -> ../../handlers/vdisk_fileio |-- nv_cache |-- o_direct |-- read_only |-- removable |-- resync_size |-- rotational |-- size_mb |-- t10_dev_id |-- thin_provisioned |-- threads_num |-- threads_pool_type |-- tst |-- type |-- usn `-- write_through Each vdisk_blockio's device has the following attributes in /sys/kernel/scst_tgt/devices/device_name: blocksize, filename, nv_cache, read_only, removable, resync_size, rotational, size_mb, t10_dev_id, thin_provisioned, gen_tp_soft_threshold_reached_UA, threads_num, threads_pool_type, tst, type, usn. See above description of those parameters. Each vdisk_nullio's device has the following attributes in /sys/kernel/scst_tgt/devices/device_name: blocksize, read_only, removable, size_mb, t10_dev_id, threads_num, threads_pool_type, type, tst, usn, dummy. See above description of those parameters. Each vcdrom's device has the following attributes in /sys/kernel/scst_tgt/devices/device_name: filename, size_mb, t10_dev_id, threads_num, threads_pool_type, type, usn, tst. See above description of those parameters. Exception is filename attribute. For vcdrom it is writable. Writing to it allows to virtually insert or change virtual CD media in the virtual CDROM device. For example: - echo "/image.iso" >/sys/kernel/scst_tgt/devices/cdrom/filename - will insert file /image.iso as virtual media to the virtual CDROM cdrom. - echo "" >/sys/kernel/scst_tgt/devices/cdrom/filename - will remove "media" from the virtual CDROM cdrom. Additionally VDISK handler has module parameter "num_threads", which specifies count of I/O threads for each FILEIO VDISK's or VCDROM device. If you have a workload, which tends to produce rather random accesses (e.g. DB-like), you should increase this count to a bigger value, like 32. If you have a rather sequential workload, you should decrease it to a lower value, like number of CPUs on the target or even 1. Due to some limitations of Linux I/O subsystem, increasing number of I/O threads too much leads to sequential performance drop, especially with deadline scheduler, so decreasing it can improve sequential performance. The default provides a good compromise between random and sequential accesses. You shouldn't be afraid to have too many VDISK I/O threads if you have many VDISK devices. Kernel threads consume very little amount of resources (several KBs) and only necessary threads will be used by SCST, so the threads will not trash your system. CAUTION: If you partitioned/formatted your device with block size X, *NEVER* ======== ever try to export and then mount it (even accidentally) with another block size. Otherwise you can *instantly* damage it pretty badly as well as all your data on it. Messages on initiator like: "attempt to access beyond end of device" is the sign of such damage. Moreover, if you want to compare how well different block sizes work for you, you **MUST** EVERY TIME AFTER CHANGING BLOCK SIZE **COMPLETELY** **WIPE OFF** ALL THE DATA FROM THE DEVICE. In other words, THE **WHOLE** DEVICE **MUST** HAVE ONLY **ZEROS** AS THE DATA AFTER YOU SWITCH TO NEW BLOCK SIZE. Switching block sizes isn't like switching between FILEIO and BLOCKIO, after changing block size all previously written with another block size data MUST BE ERASED. Otherwise you will have a full set of very weird behaviors, because blocks addressing will be changed, but initiators in most cases will not have a possibility to detect that old addresses written on the device in, e.g., partition table, don't refer anymore to what they are intended to refer. IMPORTANT: Some disk and partition table management utilities don't support ========= block sizes >512 bytes, therefore make sure that your favorite one supports it. Currently only cfdisk is known to work only with 512 bytes blocks, other utilities like fdisk on Linux or standard disk manager on Windows are proved to work well with non-512 bytes blocks. Note, if you export a disk file or device with some block size, different from one, with which it was already partitioned, you could get various weird things like utilities hang up or other unexpected behavior. Hence, to be sure, zero the exported file or device before the first access to it from the remote initiator with another block size. On Window initiator make sure you "Set Signature" in the disk manager on the imported from the target drive before doing any other partitioning on it. After you successfully mounted a file system over non-512 bytes block size device, the block size stops matter, any program will work with files on such file system. Dealing with massive logs ------------------------- If you want to enable using "trace_level" file logging levels, which produce a lot of events, like "debug", to not loose logged events you should also: * Increase in .config of your kernel CONFIG_LOG_BUF_SHIFT variable to much bigger value, then recompile it. For example, value 25 will provide good protection from logging overflow even under high volume of logging events. To use it you will need to modify the maximum allowed value for CONFIG_LOG_BUF_SHIFT in the corresponding Kconfig file to 25 as well. * Change in your /etc/syslog.conf or other config file of your favorite logging program to store kernel logs in async manner. For example, you can add in rsyslog.conf line "kern.info -/var/log/kernel" and add "kern.none" in line for /var/log/messages, so the resulting line would looks like: "*.info;kern.none;mail.none;authpriv.none;cron.none /var/log/messages" Persistent Reservations ----------------------- SCST implements Persistent Reservations with full set of capabilities, including "Persistence Through Power Loss". The "Persistence Through Power Loss" data are saved in /var/lib/scst/pr with files with names the same as the names of the corresponding devices. Also this directory contains backup versions of those files with suffix ".1". Those backup files are used in case of power or other failure to prevent Persistent Reservation information from corruption during update. It is safe to assume that each of those files can be up to 1KB big. The "Persistence Through Power Loss" feature is not available in the procfs build, because the SCST proc interface doesn't allow to keep persistent Relative Target IDs of each target between reboots/reloads (they are load and initialization order dependent). The Persistent Reservations available on all transports implementing get_initiator_port_transport_id() callback. Transports not implementing this callback will act in one of 2 possible scenarios ("all or nothing"): 1. If a device has such transport connected and doesn't have persistent reservations, it will refuse Persistent Reservations commands as if it doesn't support them. 2. If a device has persistent reservations, all initiators newly connecting via such transports will not see this device. After all persistent reservations from this device are released, upon reconnect the initiators will see it. ALUA Support ------------ SCST supports both implicit and explicit asymmetric logical unit access (ALUA). ALUA is a feature defined by the ANSI T10 SCSI committee. It allows a target to tell the initiator which path to use in a multipath setup plus, in the explicit case, control state of each path via SET TARGET PORT GROUPS SCSI command. The redundant paths between initiator and target can be used either for redundancy or for load sharing purposes. The target can either be a single target system running SCST with multiple communication interfaces or two target systems each running SCST and configured in a high availability setup. In the SPC-4 standard the following concepts are defined related to ALUA: * Relative target port ID. A number between 1 and 65535 that uniquely identifies a target port. These numbers must be unique over the target as a whole, even if that target consists of multiple systems each running SCST. * Target port group asymmetric access state. One of active/optimized, active/non-optimized, standby, unavailable, logical block dependent or offline. The access state of a port defines which (if any) SCSI commands will be processed by the target port. * Target port preference indicator. This indicator is additional information next to the asymmetric access state that is provided by the target to an initiator and that may impact the decision taken by the initiator about which path that will be chosen. More detailed information about ALUA can be found in section 5.11.2 of the ANSI T10 standard called SPC-4. ALUA support in SCST .................... SCST allows to define ALUA settings for each unique combination of SCST device and SCST target. An initiator however queries ALUA settings by sending an appropriate SCSI command to a specific LUN of an SCST target. Each such LUN maps uniquely to an SCST device. For hardware SCST target drivers, e.g. ib_srpt, there is a one-to-one correspondence between SCST target and SCSI target port. With other SCST targets, e.g. iSCSI-SCST, by default the only relationship between SCST targets and SCSI target ports is that all SCST targets defined on a system are visible via all SCSI target ports. See also the iSCSI-SCST documentation about the allowed_portal attribute for information about how to associate iSCSI targets with a single physical interface. Notes: - In a H.A. setup it is the responsibility of the user to synchronize ALUA information between the individual systems running SCST. There are no provisions in SCST to exchange ALUA information automatically between individual systems. - In order to support H.A. setups it is possible to let one SCST system report information about target ports present in other SCST systems. - With SCST, and certainly in a H.A. setup, it is possible to configure ALUA such that an initiator receives information that is not standard compliant, e.g. setting all target ports in the offline state. It is the responsibility of the user to make sure that the information queried by an initiator is consistent independent of the LUN and the target port used by the initiator to query this information. - Before building a H.A. setup consisting of two or more SCST systems one should evaluate whether it's acceptable that persistent reservation commands, SCSI task management commands and MODE SELECT commands will only be processed by a single node instead of being processed by all nodes. Configuring ALUA in SCST ........................ SCST allows to configure the following settings related to ALUA for each unique combination of SCST target and virtual SCST device (vdisk_fileio, vdisk_blockio, vcdrom, ...): * The target port group asymmetric access state. SCST supports all ALUA port states except logical block dependent. * The preference indicator for a target port group. * The relative target port ID associated with the SCST target. It is possible to configure the following ALUA-related information via the sysfs interface of SCST: * Device groups, where each device group has a name and contains zero or more SCST devices. If a device group contains only a single SCST device, the name of the group may be identical to the device name. See also /sys/kernel/scst_tgt/device_groups/mgmt. * Which devices are inside a device group. See also /sys/kernel/scst_tgt/device_groups//devices/mgmt. * Target groups, where each target group has a name and contains zero or more SCST target names. See also /sys/kernel/scst_tgt/device_groups//target_groups/mgmt. * Target port group identifier. This is a number in the range 0..65535 and is called the TARGET PORT GROUP in SPC-4. See also /sys/kernel/scst_tgt/device_groups//target_groups//group_id. * Target port group preference indicator. This is a boolean value called the PREF bit in SPC-4. See also /sys/kernel/scst_tgt/device_groups//target_groups//preferred. * Target port group state name. One of active, nonoptimized, standby, unavailable, offline or transitioning. See also /sys/kernel/scst_tgt/device_groups//target_groups//state. * Target group contents - zero or more target names. The target names either exist on the local system or on a remote system in a H.A. setup. For target names that refer to SCST targets on another system only the relative target port identifier matters, not the assigned name. See also /sys/kernel/scst_tgt/device_groups//target_groups//mgmt. * Relative target identifier. See also /sys/kernel/scst_tgt/device_groups//target_groups///rel_tgt_id. The steps involved in configuring ALUA are: * Identify the SCST devices that will always share the same ALUA settings and state. Assign a name to each such group of SCST devices. If a device group only contains a single device, the group name may be identical to the device name. * Configure that device group in SCST via sysfs. * Identify the SCSI target ports that will always share the same ALUA settings and state. Assign a name, a group ID and preference indicator to each such SCSI target port group. * Configure the target port group information in SCST via sysfs. * Identify all SCST targets that can be accessed via a target port group. * Assign all these SCST target names to the target group via sysfs. * Assign a relative target port identifier to each target. As an example, in a H.A. setup with two systems each having one InfiniBand HCA controlled by the ib_srpt driver and where each system exports two LUNs the following configuration can be used in scst.conf on both systems: DEVICE_GROUP dgroup1 { DEVICE disk01 TARGET_GROUP tgroup1 { group_id 256 preferred 1 state active TARGET fe80:0000:0000:0000:0002:c903:00fa:b7e1 { rel_tgt_id 1 } } TARGET_GROUP tgroup2 { group_id 257 state standby TARGET fe80:0000:0000:0000:0002:c903:00fa:b7f2 { rel_tgt_id 2 } } } DEVICE_GROUP dgroup2 { DEVICE disk02 TARGET_GROUP tgroup1 { group_id 258 state standby TARGET fe80:0000:0000:0000:0002:c903:00fa:b7e1 { rel_tgt_id 1 } } TARGET_GROUP tgroup2 { group_id 259 preferred 1 state active TARGET fe80:0000:0000:0000:0002:c903:00fa:b7f2 { rel_tgt_id 2 } } } Explicit ALUA ............. To enable explicit ALUA you need in addition to the above settings set expl_alua device attribute to 1 (by default it is 0). Also you need to run stpgd and supply to it path to a script or program, which will perform actual path state switching on SET TARGET PORT GROUPS command, for instance, by calling drbdadm. For more information see stpgd README as well as sample script scst_on_stpg. DRBD and other replication/failover SW compatibility .................................................... DRBD as well as other replication/failover SW does not allow to open its device on the secondary as well as does not allow to perform primary to secondary transition, if this device is open. SCST BLOCKIO handler has all the necessary support for such behavior. If you write new ALUA state in the "state" attribute, SCST BLOCKIO handler before transition closes the open devices' handles and after transition reopens them, if the new state is active or nonoptimized. Thus, the recommended implicit ALUA state change procedure for primary to secondary transition is: 1. Block all involved SCST devices using "block" sysfs attribute (see above). Wait until the blocking finished. 2. Change the ALUA state to "transitioning". At this moment all open file handles will be closed. 3. Perform the DRBD or other replication/failover SW state transition 4. Change the ALUA state to your desired secondary state. 5. Unblock the blocked on step 1 devices. Optionally, if your initiators support Transitioning ALUA state, for more responsive behavior the blocked devices can be unblocked immediately after step (2). However, not all initiators correctly behave, if they receive ASYMMETRIC STATE TRANSITION sense. For the secondary to primary transition procedure is similar. In case of explicit ALUA, SCST automatically performs the necessary devices blocking around sending SCST_EVENT_STPG_USER_INVOKE event. Checking the Target Configuration ................................. One way to verify the ALUA configuration from a Linux initiator is via the commands provided in the sg3_utils package. The first step is to verify whether for a certain LUN ALUA has been configured on the target. This is possible by checking whether the TPGS=1 text appears in the sg_inq output, where /dev/sdb is a device node created by the ib_srp initiator: # sg_inq /dev/sdb standard INQUIRY: PQual=0 Device_type=0 RMB=0 version=0x05 [SPC-3] [AERC=0] [TrmTsk=0] NormACA=0 HiSUP=1 Resp_data_format=2 SCCS=0 ACC=0 TPGS=1 3PC=0 Protect=0 BQue=0 EncServ=0 MultiP=0 [MChngr=0] [ACKREQQ=0] Addr16=1 [RelAdr=0] WBus16=0 Sync=0 Linked=0 [TranDis=0] CmdQue=1 [SPI: Clocking=0x0 QAS=0 IUS=0] length=66 (0x42) Peripheral device type: disk Vendor identification: SCST_FIO Product identification: disk01 Product revision level: 300 Unit serial number: 27cddc71 The next step is to verify the target group configuration. That is possible by verifying whether the output of the sg_rtpg command matches the values configured on the target: # sg_rtpg /dev/sdb Report target port groups: target port group id : 0x100 , Pref=1 target port group asymmetric access state : 0x00 T_SUP : 0, O_SUP : 0, LBD_SUP : 0, U_SUP : 1, S_SUP : 1, AN_SUP : 1, AO_SUP : 1 status code : 0x02 vendor unique status : 0x00 target port count : 01 Relative target port ids: 0x01 target port group id : 0x101 , Pref=0 target port group asymmetric access state : 0x02 T_SUP : 0, O_SUP : 0, LBD_SUP : 0, U_SUP : 1, S_SUP : 1, AN_SUP : 1, AO_SUP : 1 status code : 0x02 vendor unique status : 0x00 target port count : 01 Relative target port ids: 0x02 The relative target port ID and the target port group ID for a certain path can be queried e.g. as follows: # sg_vpd -p di /dev/sdb Device Identification VPD page: Addressed logical unit: designator type: T10 vendor identification, code set: ASCII vendor id: SCST_FIO vendor specific: 27cddc71-disk01 designator type: EUI-64 based, code set: Binary 0x3237636464633731 Target port: designator type: Relative target port, code set: Binary Relative target port: 0x1 designator type: Target port group, code set: Binary Target port group: 0x100 Initiator Support ................. On Linux systems ALUA support is provided by the scsi_dh_alua kernel driver in combination with the user space multipathd daemon. You will have to modify at least the following in /etc/multipath.conf to enable ALUA: * hardware_handler "1 alua" * prio alua * path_grouping_policy group_by_prio * path_checker tur Notes: - Newer versions of multipathd support a parameter called "detect_prio". It can be more convenient to enable this parameter instead of setting the parameter "prio" to "alua" for only those LUNs that support ALUA. - Older versions of multipathd (e.g. RHEL 5 and SLES 10 SP1) need 'prio_callout "/sbin/mpath_prio_alua /dev/%n"' instead of 'prio alua'. # multipath -ll 23237636464633731 dm-3 SCST_FIO,disk01 size=1.0G features='3 queue_if_no_path pg_init_retries 50' hwhandler='1 alua' wp=rw |-+- policy='service-time 0' prio=1 status=active | `- 10:0:0:0 sdd 8:48 active ready running `-+- policy='service-time 0' prio=130 status=enabled `- 11:0:0:0 sde 8:64 active ready running 23133326137346538 dm-4 SCST_FIO,disk02 size=1.0G features='3 queue_if_no_path pg_init_retries 50' hwhandler='1 alua' wp=rw |-+- policy='service-time 0' prio=130 status=active | `- 10:0:0:2 sdn 8:208 active ready running `-+- policy='service-time 0' prio=1 status=enabled `- 11:0:0:2 sdp 8:240 active ready running The following information can be derived from the above output: * That the hardware handler (hw_handler) has been set to "1 alua". * That multipathd created two priority groups - one with priority 1 and one with priority 130. * That the SRP path with SCSI host number 10 will be used for communication with LUN "disk01" and that the SRP path with SCSI host number 11 will be used for communication with LUN "disk02". More information about how to configure the device mapper and the scsi_dh_alua driver can be found in the manual of your Linux distribution ("man multipath.conf", "man multipath" and "man multipathd"). Windows initiator systems support ALUA from Windows Server 2008 on. For more information about ALUA support in Windows Server, see also: * Microsoft, Windows Server 2008 R2 Multipath I/O Overview, MSDN (http://technet.microsoft.com/en-us/library/cc725907.aspx). * Microsoft, Multipathing Support in Windows Server 2008, July 2008, MSDN (http://blogs.msdn.com/b/san/archive/2008/07/27/multipathing-support-in-windows-server-2008.aspx). * Microsoft, ALUA MPIO Logo Test, MSDN (http://msdn.microsoft.com/en-us/library/gg607458%28v=vs.85%29.aspx). VAAI ---- SCST supports all 3 VAAI SCSI commands: WRITE SAME, COMPARE AND WRITE (ATS) and EXTENDED COPY. Additionally, it supports not directly related to VAAI Thin Provisioning capabilities, particularly, UNMAP SCSI commands, WRITE SAME with UNMAP bit as well as thin provisioning related devices' sysfs attributes (see above). In some cases dev handlers should perform some manual actions to fully benefit from SCST VAAI implementation. Those actions described in the implementation notes below. For vdisk and fileio_tgt handlers they have already been implemented. IMPORTANT: To use EXTENDED COPY command between LUNs (datastores) they all ========= MUST have the same PRODUCT IDENTIFICATION INQUIRY field. By default, to simplify remote devices identification, SCST uses vdisk names as PRODUCT IDENTIFICATION, so SCST devices look differently from the initiators. However, for some reasons, VMware does not use EXTENDED COPY between LUNs with different PRODUCT IDENTIFICATION. Thus, to be able to use full VAAI in your VMware setups you must manually set PRODUCT IDENTIFICATION for all your VMware LUNs to the same value, for instance, "SCST", via using "prod_id" attribute. It could be done either by adding "prod_id" attribute to scstadmin scst.conf, or by directly writing to SCST sysfs attribute. For example: HANDLER vdisk_blockio { DEVICE blockio1 { filename /dev/sda5 prod_id SCST } or echo SCST >/sys/kernel/scst_tgt/devices/blockio1/prod_id correspondingly. Note, this prod_id modification must be done on all datastores BEFORE VMware connects to them. Implementation notes .................... WRITE SAME ~~~~~~~~~~ WRITE SAME command supports 2 modes: 1. Manual writing mode. In this mode WRITE SAME generates a set of internal WRITE(16) SCSI commands to perform requested writing. 2. Remap mode. In this mode a dev handler, if supported, can remap being written blocks to a single block and then tell SCST to manually write parts of the requested area, which for some reason can not be remapped. In both cases dev handlers should call from WRITE SAME command handler scst_write_same() function. This function as the second argument gets array of descriptors where to write the requested block of data. Last element in this array must have len 0. If this argument is NULL, then the whole area will be manually written by SCST. This value should be used by dev handlers not supporting remapping blocks. User space dev handlers should use SCST_EXEC_REPLY_DO_WRITE_SAME reply_type of SCST_USER_EXEC subcommand. See scst_user doc for more info. COMPARE AND WRITE ~~~~~~~~~~~~~~~~~ COMPARE AND WRITE implemented by SCST a set of read, compare and write actions done in atomic manner against affected blocks as well as regular RESERVE SCSI commands. Particularly, COMPARE AND WRITE doesn't need any queue flushing and unlimited number of COMPARE AND WRITE commands on different blocks can be executed simultaneously. The read and write actions implemented as generation of internal READ(16) and WRITE(16) SCSI commands. COMPARE AND WRITE command is completely transparent to dev handlers (they only see the corresponding READ(16) and WRITE(16) commands), so doesn't require any manual actions from them. EXTENDED COPY ~~~~~~~~~~~~~ SCST implements EXTENDED COPY via internal Copy Manager target. This target has the following specific attribute in its sysfs: - allow_not_connected_copy - if not set (default), an initiator can perform copy only between devices it has direct access to via any target/session. If set, any initiator can copy between any devices in the system. The Copy Manager has access only to those devices, for which it has LUNs in /sys/kernel/scst_tgt/targets/copy_manager/copy_manager_tgt/luns/. Devices from scst_vdisk dev handler added to it automatically upon registration, but for other devices you need to manually add LUNs there the same way as for any target driver. You can also delete any device at any time from the Copy Manager visibility by deleting the corresponding LUN from the sysfs. It might be useful during ALUA state switching. Internally SCST implements EXTENDED COPY as generation of sets of internal READ(16) and WRITE(16) SCSI commands. Dev handlers don't need any manual actions to use it. Also SCST provides for dev handlers possibility to remap blocks instead of copy them, if they support this feature. It allows them to perform EXTENDED COPY command much faster by just metadata update of their backend storage, which supposed to be nearly instantaneous. To use this feature, a dev handler should setup ext_copy_remap() callback in its struct scst_dev_type. This callback is called by SCST during EXTENDED COPY command processing to let the dev handler try to remap affected blocks at first. Upon finish, the dev handler should call scst_ext_copy_remap_done(). In case of error, the dev handler should set the corresponding sense to cmd and then also call scst_ext_copy_remap_done(cmd, NULL, 0). If dev handler is not able to remap any part of the segment, if should kmalloc(), then fill all leftover subsegments and supply them to scst_ext_copy_remap_done(). SCST then will copy the subsegments using internal copy machine, then kfree() the supplied array. If the dev handler is not able to remap the whole segment, it can simply directly supply the original segment to scst_ext_copy_remap_done(). It is highly recommended that in normal circumstances dev handlers call scst_ext_copy_remap_done() from another thread context than one where ext_copy_remap() callback was originally called, because otherwise there could be recursion in the segments processing. Hopefully, this thread context switch is natural for such potentially long operation as EXTENDED COPY. VMware and Ceph RBD space reclaim --------------------------------- VMware with VMFS5 filesystem ignores UNMAP alignment, so if you use 4MB Ceph RBD objects and VMFS5, only some discards will reclaim RBD space due to 1MB discard not often hitting the tail of objects. Thus, to have efficient ESXi space reclamation with RBD and VMFS5, you are recommended to use 1 MB object size in Ceph. See https://sourceforge.net/p/scst/mailman/message/35287598 thread for details. Caching ------- By default for performance reasons VDISK FILEIO devices use write back caching policy. Generally, write back caching is safe for use and danger of it is greatly overestimated, because most modern (especially, Enterprise level) applications are well prepared to work with write back cached storage. Particularly, such are all transactions-based applications. Those applications flush cache to completely avoid ANY data loss on a crash or power failure. For instance, journaled file systems flush cache on each meta data update, so they survive power/hardware/software failures pretty well. Since locally on initiators write back caching is always on, if an application cares about its data consistency, it does flush the cache when necessary or on any write, if open files with O_SYNC. If it doesn't care, it doesn't flush the cache. As soon as the cache flushes propagated to the storage, write back caching on it doesn't make any difference. If application doesn't flush the cache, it's doomed to loose data in case of a crash or power failure doesn't matter where this cache located, locally or on the storage. To illustrate that consider, for example, a user who wants to copy /src directory to /dst directory reliably, i.e. after the copy finished no power failure or software/hardware crash could lead to a loss of the data in /dst. There are 2 ways to achieve this. Let's suppose for simplicity cp opens files for writing with O_SYNC flag, hence bypassing the local cache. 1. Slow. Make the device behind /dst working in write through caching mode and then run "cp -a /src /dst". 2. Fast. Let the device behind /dst working in write back caching mode and then run "cp -a /src /dst; sync". The reliability of the result is the same, but it's much faster than (1). Nobody would care if a crash happens during the copy, because after recovery simply leftovers from the not completed attempt would be deleted and the operation would be restarted from the very beginning. So, you can see in (2) there is no danger of ANY data loss from the write back caching. Moreover, since on practice cp doesn't open files for writing with O_SYNC flag, to get the copy done reliably, sync command must be called after cp anyway, so enabling write back caching wouldn't make any difference for reliability. Also you can consider it from another side. Modern HDDs have at least 16MB of cache working in write back mode by default, so for a 10 drives RAID it is 160MB of a write back cache. How many people are happy with it and how many disabled write back cache of their HDDs? Almost all and almost nobody correspondingly? Moreover, many HDDs lie about state of their cache and report write through while working in write back mode. They are also successfully used. Note, Linux I/O subsystem guarantees to propagated cache flushes to the storage only using data protection barriers, which usually turned off by default (see http://lwn.net/Articles/283161). Without barriers enabled Linux doesn't provide a guarantee that after sync()/fsync() all written data really hit permanent storage. They can be stored in the cache of your backstorage devices and, hence, lost on a power failure event. Thus, ever with write-through cache mode, you still either need to enable barriers on your backend file system on the target (for direct /dev/sdX devices this is, indeed, impossible), or need a good UPS to protect yourself from not committed data loss. Some info about barriers from the XFS point of view could be found at http://xfs.org/index.php/XFS_FAQ#Write_barrier_support. On Linux initiators for Ext3 and ReiserFS file systems the barrier protection could be turned on using "barrier=1" and "barrier=flush" mount options correspondingly. You can check if the barriers turn on or off by looking in /proc/mounts. Windows and, AFAIK, other UNIX'es don't need any special explicit options and do necessary barrier actions on write-back caching devices by default. To limit this data loss with write back caching you can use files in /proc/sys/vm to limit amount of unflushed data in the system cache. If you for some reason have to use VDISK FILEIO devices in write through caching mode, don't forget to disable internal caching on their backend devices or make sure they have additional battery or supercapacitors power supply on board. Otherwise, you still on a power failure would loose all the unsaved yet data in the devices internal cache. Note, on some real-life workloads write through caching might perform better, than write back one with the barrier protection turned on. Errors caching .............. When using virtual device in FILEIO mode, the Linux page cache comes into picture. The negative side of it is that it's sometimes also caching errored pages. That is, if the underlying file experiences IO errors, those errors might be cached by the Linux page cache. As a result, even when the underlying file recovers and stops failing IOs, the initiator may still hit IO errors returned by the Linux page cache, until the cache re-reads the errored pages (usually it happens pretty soon, but not immediately). To make sure that cached pages are dropped, one of the following can be done: - Detach the SCSI virtual device (del_device) and re-attach it (add_device). This should evict all the cached pages, unless somebody else holds the same "filename" opened. - Issue a BLKFLSBUF ioctl to the same "filename" you provided for "add_device". For the second option, a rudimentary C code is required: fd = open(filename, O_RDWR); if (fd < 0) { err = errno; ... } else { err = ioctl(fd, BLKFLSBUF); if (err < 0) { err = errno; ... } close(fd); } BLOCKIO VDISK mode ------------------ This module works best for these types of scenarios: 1) Data that are not aligned to 4K sector boundaries and <4K block sizes are used, which is normally found in virtualization environments where operating systems start partitions on odd sectors (Windows and it's sector 63). 2) Large block data transfers normally found in database loads/dumps and streaming media. 3) Advanced relational database systems that perform their own caching which prefer or demand direct IO access and, because of the nature of their data access, can actually see worse performance with non-discriminate caching. 4) Multiple layers of targets were the secondary and above layers need to have a consistent view of the primary targets in order to preserve data integrity which a page cache backed IO type might not provide reliably. Also it has an advantage over FILEIO that it doesn't copy data between the system cache and the commands data buffers, so it saves a considerable amount of CPU power and memory bandwidth. IMPORTANT: Since data in BLOCKIO and FILEIO modes are not consistent between ========= each other, if you try to use a device in both those modes simultaneously, you will almost instantly corrupt your data on that device. IMPORTANT: Some kernels starting from 2.6.32 have a problem, which ========= prevents BLOCKIO from working correctly with RAID5/DM. See http://lkml.org/lkml/2010/7/28/315. That problem was fixed in 2.6.32.19, 2.6.34.4, 2.6.35.2 and 2.6.36-rc1. It is strongly recommended to not use affected kernels with BLOCKIO. IMPORTANT: In SCST 1.x BLOCKIO worked by default in NV_CACHE mode, when ========= each device reported to remote initiators as having write through caching. But if your backend block device has internal write back caching it might create a possibility for data loss of the cached in the internal cache data in case of a power failure. Starting from SCST 2.0 BLOCKIO works by default in non-NV_CACHE mode, when each device reported to remote initiators as having write back caching, and synchronizes the internal device's cache on each SYNCHRONIZE_CACHE command from the initiators. It might lead to some *PERFORMANCE LOSS*, so if you are are sure in your power supply and want to restore the 1.x behavior, your should recreate your BLOCKIO devices in NV_CACHE mode. Pass-through mode ----------------- In the pass-through mode (i.e. using the pass-through device handlers scst_disk, scst_tape, etc) SCSI commands, coming from remote initiators, are passed to local SCSI devices on target as is, without any modifications. SCST supports 1 to many pass-through, when several initiators can safely connect a single pass-through device (a tape, for instance). For such cases SCST emulates all the necessary functionality. In the sysfs interface all real SCSI devices are listed in /sys/kernel/scst_tgt/devices in form host:channel:id:lun numbers, for instance 1:0:0:0. The recommended way to match those numbers to your devices is use of lsscsi utility. Each pass-through dev handler has in its root subdirectory /sys/kernel/scst_tgt/handlers/handler_name, e.g. /sys/kernel/scst_tgt/handlers/dev_disk, "mgmt" file. It allows the following commands. They can be sent to it using, e.g., echo command. - "add_device" - this command assigns SCSI device with host:channel:id:lun numbers to this dev handler. echo "add_device 1:0:0:0" >/sys/kernel/scst_tgt/handlers/dev_disk/mgmt will assign SCSI device 1:0:0:0 to this dev handler. - "del_device" - this command unassigns SCSI device with host:channel:id:lun numbers from this dev handler. As usually, on read the "mgmt" file returns small help about available commands. You need to manually assign each your real SCSI device to the corresponding pass-through dev handler using the "add_device" command, otherwise the real SCSI devices will not be visible remotely. The assignment isn't done automatically, because it could lead to the pass-through dev handlers load and initialization problems if any of the local real SCSI devices are malfunctioning. As any other hardware, the local SCSI hardware can not handle commands with amount of data and/or segments count in scatter-gather array bigger some values. Therefore, when using the pass-through mode you should note that values for maximum number of segments and maximum amount of transferred data (max_sectors) for each SCSI command on devices on initiators can not be bigger, than corresponding values of the corresponding SCSI devices on the target. Otherwise you will see symptoms like small transfers work well, but large ones stall and messages like: "Unable to complete command due to SG IO count limitation" are printed in the kernel logs. You can't control from the user space limit of the scatter-gather segments, but for block devices usually it is sufficient if you set on the initiators /sys/block/DEVICE_NAME/queue/max_sectors_kb in the same or lower value as in /sys/block/DEVICE_NAME/queue/max_hw_sectors_kb for the corresponding devices on the target. For not-block devices SCSI commands are usually generated directly by applications, so, if you experience large transfers stalls, you should check documentation for your application how to limit the transfer sizes. Another way to solve this issue is to build SG entries with more than 1 page each. See the following patch as an example: http://scst.sourceforge.net/sgv_big_order_alloc.diff User space mode using scst_user dev handler ------------------------------------------- User space program fileio_tgt uses interface of scst_user dev handler and allows to see how it works in various modes. Fileio_tgt provides mostly the same functionality as scst_vdisk handler with the most noticeable difference that it supports O_DIRECT mode. O_DIRECT mode is basically the same as BLOCKIO, but also supports files, so for some loads it could be significantly faster, than the regular FILEIO access. All the words about BLOCKIO from above apply to O_DIRECT as well. See fileio_tgt's README file for more details. Performance ----------- SCST from the very beginning has been designed and implemented to provide the best possible performance. Since there is no "one fit all" the best performance configuration for different setups and loads, SCST provides extensive set of settings to allow to tune it for the best performance in each particular case. You don't have to necessary use those settings. If you don't, SCST will do very good job to autotune for you, so the resulting performance will, in average, be better (sometimes, much better) than with other SCSI targets. But in some cases you can by manual tuning improve it even more. Before doing any performance measurements note that performance results are very much dependent from your type of load, so it is crucial that you choose access mode (FILEIO, BLOCKIO, O_DIRECT, pass-through), which suits your needs the best. In order to get the maximum performance you should: 1. For SCST: - Disable in Makefile and scst.h CONFIG_SCST_STRICT_SERIALIZING, CONFIG_SCST_EXTRACHECKS, CONFIG_SCST_TRACING, CONFIG_SCST_DEBUG*, CONFIG_SCST_STRICT_SECURITY, CONFIG_SCST_MEASURE_LATENCY 2. For target drivers: - Disable in Makefiles CONFIG_SCST_EXTRACHECKS, CONFIG_SCST_TRACING, CONFIG_SCST_DEBUG* 3. For device handlers, including VDISK: - Disable in Makefile CONFIG_SCST_TRACING and CONFIG_SCST_DEBUG. Note, by disabling CONFIG_SCST_TRACING and CONFIG_SCST_DEBUG you are disabling many useful SCST diagnostic messages, which can significantly help in many troubleshooting cases. So, if you may consider to keep CONFIG_SCST_TRACING, its performance impact is very limited. IMPORTANT: The development version of SCST in the SVN is optimized for ========= development and bug hunting, not for performance. This means it is MUCH slower (multiple times). To reconfigure SCST for release you should run "make 2release" command in the root of your source code (e.g. trunk/). It will set the above options as needed. The only option it doesn't set is CONFIG_SCST_TEST_IO_IN_SIRQ, so, if needed, you should change it manually. There is also so called "performance" build mode, which you can activate by "make 2perf" command. The only difference it has comparing to release build mode is disabled CONFIG_SCST_TRACING option. Because of that, you won't be able to see many important SCST run time logging messages. This mode is intended to evaluate impact of CONFIG_SCST_TRACING on performance and not recommended for production. IMPORTANT: You can't use debug SCST drivers with non-debug SCST core. ========= So, after disabling both CONFIG_SCST_TRACING and CONFIG_SCST_DEBUG for SCST core you have to disable them for all SCST drivers you are using as well. 4. Make sure you have io_grouping_type option set correctly, especially in the following cases: - Several initiators share your target's backstorage. It can be a shared LU using some cluster FS, like VMFS, as well as can be different LUs located on the same backstorage (RAID array). For instance, if you have 3 initiators and each of them using its own dedicated FILEIO device file from the same RAID-6 array on the target. In this case for the best performance you should have io_grouping_type option set in value "never" in all the LUNs' targets and security groups. - Your initiator connected to your target in MPIO mode. In this case for the best performance you should: * Either connect all the sessions from the initiator to a single target or security group and have io_grouping_type option set in value "this_group_only" in the target or security group, * Or, if it isn't possible to connect all the sessions from the initiator to a single target or security group, assign the same numeric io_grouping_type value for each target/security group this initiator connected to. The exact value itself doesn't matter, important only that all the targets/security groups use the same value. Don't forget, io_grouping_type makes sense only if you use CFQ I/O scheduler on the target and for devices with threads_num >= 0 and, if threads_num > 0, with threads_pool_type "per_initiator". You can check if in your setup io_grouping_type set correctly as well as if the "auto" io_grouping_type value works for you by tests like the following: - For not MPIO case you can run single thread sequential reading, e.g. using buffered dd, from one initiator, then run the same single thread sequential reading from the second initiator in parallel. If io_grouping_type is set correctly the aggregate throughput measured on the target should only slightly decrease as well as all initiators should have nearly equal share of it. If io_grouping_type is not set correctly, the aggregate throughput and/or throughput on any initiator will decrease significantly, in 2 times or even more. For instance, you have 80MB/s single thread sequential reading from the target on any initiator. When then both initiators are reading in parallel you should see on the target aggregate throughput something like 70-75MB/s with correct io_grouping_type and something like 35-40MB/s or 8-10MB/s on any initiator with incorrect. - For the MPIO case it's quite easier. With incorrect io_grouping_type you simply won't see performance increase from adding the second session (assuming your hardware is capable to transfer data through both sessions in parallel), or can even see a performance decrease. 5. If you are going to use your target in an VM environment, for instance as a shared storage with VMware, make sure all your VMs connected to the target via *separate* sessions. For instance, for iSCSI it means that each VM has own connection to the target, not all VMs connected using a single connection. You can check it using SCST sysfs interface. For other transports you should use available facilities, like NPIV for Fibre Channel, to make separate sessions for each VM. If you miss it, you can greatly loose performance of parallel access to your target from different VMs. This isn't related to the case if your VMs are using the same shared storage, like with VMFS, for instance. In this case all your VM hosts will be connected to the target via separate sessions, which is enough. 6. For other target and initiator software parts: - Make sure you applied on your kernel all available SCST patches. If for your kernel version this patch doesn't exist, it is strongly recommended to upgrade your kernel to version, for which this patch exists. - Don't enable debug/hacking features in the kernel, i.e. use them as they are by default. - The default kernel read-ahead and queuing settings are optimized for locally attached disks, therefore they are not optimal if they attached remotely (SCSI target case), which sometimes could lead to unexpectedly low throughput. You should increase read-ahead size to at least 512KB or even more on all initiators and the target. You should also limit on all initiators maximum amount of sectors per SCSI command. This tuning is also recommended on targets with large read-ahead values. To do it on Linux, run: echo “64” > /sys/block/sdX/queue/max_sectors_kb where specify instead of X your imported from target device letter, like 'b', i.e. sdb. To increase read-ahead size on Linux, run: blockdev --setra N /dev/sdX where N is a read-ahead number in 512-byte sectors and X is a device letter like above. Note: you need to set read-ahead setting for device sdX again after you changed the maximum amount of sectors per SCSI command for that device. Note2: you need to restart SCST after you changed read-ahead settings on the target. It is a limitation of the Linux read ahead implementation. It reads RA values for each file only when the file is open and not updates them when the global RA parameters changed. Hence, the need for vdisk to reopen all its files/devices. - You may need to increase amount of requests that OS on initiator sends to the target device. To do it on Linux initiators, run echo “64” > /sys/block/sdX/queue/nr_requests where X is a device letter like above. You may also experiment with other parameters in /sys/block/sdX directory, they also affect performance. If you find the best values, please share them with us. - On the target use CFQ IO scheduler. In most cases it has performance advantage over other IO schedulers, sometimes huge (2+ times aggregate throughput increase). - It is recommended to turn the kernel preemption off, i.e. set the kernel preemption model to "No Forced Preemption (Server)". - Looks like XFS is the best filesystem on the target to store device files, because it allows considerably better linear write throughput, than ext3. 7. For hardware on target. - Make sure that your target hardware (e.g. target FC or network card) and underlying IO hardware (e.g. IO card, like SATA, SCSI or RAID to which your disks connected) don't share the same PCI bus. You can check it using lspci utility. They have to work in parallel, so it will be better if they don't compete for the bus. The problem is not only in the bandwidth, which they have to share, but also in the interaction between cards during that competition. This is very important, because in some cases if target and backend storage controllers share the same PCI bus, it could lead up to 5-10 times less performance, than expected. Moreover, some motherboard (by Supermicro, particularly) have serious stability issues if there are several high speed devices on the same bus working in parallel. If you have no choice, but PCI bus sharing, set in the BIOS PCI latency as low as possible. 8. If you use VDISK IO module in FILEIO mode, NV_CACHE option will provide you the best performance. But using it make sure you use a good UPS with ability to shutdown the target on the power failure. Baseline performance numbers you can find in those measurements: http://lkml.org/lkml/2009/3/30/283. IMPORTANT: If you use on initiator some versions of Windows (at least W2K) ========= you can't get good write performance for VDISK FILEIO devices with default 512 bytes block sizes. You could get about 10% of the expected one. This is because of the partition alignment, which is (simplifying) incompatible with how Linux page cache works, so for each write the corresponding block must be read first. Use 4096 bytes block sizes for VDISK devices and you will have the expected write performance. Actually, any OS on initiators, not only Windows, will benefit from block size max(PAGE_SIZE, BLOCK_SIZE_ON_UNDERLYING_FS), where PAGE_SIZE is the page size, BLOCK_SIZE_ON_UNDERLYING_FS is block size on the underlying FS, on which the device file located, or 0, if a device node is used. Both values are from the target. See also important notes about setting block sizes >512 bytes for VDISK FILEIO devices above. 9. In some cases, for instance working with SSD devices, which consume 100% of a single CPU load for data transfers in their internal threads, to maximize IOPS it can be needed to assign for those threads dedicated CPUs. Consider using cpu_mask attribute for devices with threads_pool_type "per_initiator" or Linux CPU affinity facilities for other threads_pool_types. No IRQ processing should be done on those CPUs. Check that using /proc/interrupts. See taskset command and Documentation/IRQ-affinity.txt in your kernel's source tree for how to assign IRQ affinity to tasks and IRQs. The reason for that is that processing of coming commands in SIRQ context might be done on the same CPUs as SSD devices' threads doing data transfers. As the result, those threads won't receive all the processing power of those CPUs and perform worse. 10. If your storage is capable of operation on hundreds of thousands IOPS level, you can use poll_us sysfs attribute to set how many us each SCST thread is polling its queue after it became empty in a hope that a new command can come. In some cases, polling can significantly increase IOPS, especially if low power states on CPU not disabled, because on high IOPS polling could be cheaper comparing to spending significant time on entering, then exiting CPU low power states + corresponding context switches. Polling is disabled by default. The recommended value to start from is 5-10 us. Then you can increase or decrease it to see if your IOPS are increasing or decreasing. Commands suspending takes too long ---------------------------------- SCST is suspending commands during some management activities like adding/deleting LUNs or devices. It is done to have lockless LUNs translation on the hot commands processing path. This brings significant performance advantage. You will see a message like "Waiting for X active commands to complete" when this wait started. But downside of it is that no new commands start executing until older ones, which had started before the suspending begun, finished. This wait can not be any longer, than the worst command latency any your initiator is seeing at this particular time. So, if this wait takes too long, in majority of cases it means that you are overloading your storage. A proper storage should have worst case latency below few hundreds of milliseconds. In this case the SCST suspending will finish in few hundreds of milliseconds at worse. Another case, when it can take too long to suspend is a hung user space device (i.e. scst_user device) not responding to any command. In this case you should kill the corresponding user space program to finish suspending. Work if target's backstorage or link is too slow ------------------------------------------------ Under high I/O load, when your target's backstorage gets overloaded, or working over a slow link between initiator and target, when the link can't serve all the queued commands on time, you can experience I/O stalls or see in the kernel log abort or reset messages. At first, consider the case of too slow target's backstorage. On some seek intensive workloads even fast disks or RAIDs, which able to serve continuous data stream on 500+ MB/s speed, can be as slow as 0.3 MB/s. Another possible cause for that can be MD/LVM/RAID on your target as in http://lkml.org/lkml/2008/2/27/96 (check the whole thread as well). Thus, in such situations simply processing of one or more commands takes too long time, hence initiator decides that they are stuck on the target and tries to recover. Particularly, it is known that the default amount of simultaneously queued commands (48) is sometimes too high if you do intensive writes from VMware on a target disk, which uses LVM in the snapshot mode. In this case value like 16 or even 8-10 depending of your backstorage speed could be more appropriate. There are 6 possible actions, which you can do to workaround or fix such issues: 1. Ignore incoming task management (TM) commands. It's fine if there are not too many of them, so average performance isn't hurt and the corresponding device isn't getting put offline, i.e. if the backstorage isn't a way too slow. 2. Decrease /sys/block/sdX/device/queue_depth on the initiator in case if it's Linux (see below how) or/and SCST_MAX_TGT_DEV_COMMANDS constant in scst_priv.h file until you stop seeing incoming TM commands. ISCSI-SCST driver also has its own iSCSI specific parameter for that, see its README file. To decrease device queue depth on Linux initiators you can run command: # echo Y >/sys/block/sdX/device/queue_depth where Y is the new number of simultaneously queued commands, X - your imported device letter, like 'a' for sda device. There are no special limitations for Y value, it can be any value from 1 to possible maximum (usually, 32), so start from dividing the current value on 2, i.e. set 16, if /sys/block/sdX/device/queue_depth contains 32. 3. Increase the corresponding timeout on the initiator. For Linux it is located in /sys/devices/platform/host*/session*/target*:0:0/*:0:0:1/timeout. It can be done automatically by an udev rule. For instance, the following rule will increase it to 300 seconds: SUBSYSTEM=="scsi", KERNEL=="[0-9]*:[0-9]*", ACTION=="add", ATTR{type}=="0|7|14", ATTR{timeout}="300" By default, this timeout is 30 or 60 seconds, depending on your distribution. 4. Try to avoid such seek intensive workloads. 5. Increase speed of the target's backstorage. 6. Implement in SCST QoS, so queue depth size on the target is dynamically adjusted, hence worst case initiator seen latencies are controlled. Next, consider the case of too slow link between initiator and target, when the initiator tries to simultaneously push N commands to the target over it. In this case time to serve those commands, i.e. send or receive data for them over the link, can be more, than timeout for any single command, hence one or more commands in the tail of the queue can not be served on time less than the timeout, so the initiator will decide that they are stuck on the target and will try to recover. To workaround/fix this issue in this case you can use ways 1, 2, 3 above or (7): increase speed of the link between target and initiator. Note, that logged messages about QUEUE_FULL status are quite different by nature. This is a normal work, just SCSI flow control in action. Simply don't enable "mgmt_minor" logging level, or, alternatively, if you are confident in the worst case performance of your back-end storage or initiator-target link, you can increase SCST_MAX_TGT_DEV_COMMANDS in scst_priv.h to 64. Usually initiators don't try to push more commands on the target. Obsolete /proc interface ------------------------ Also for communications with user space programs SCST provides proc-based interface in /proc/scsi_tgt directory. This interface is available in the procfs build only. Starting from version 2.0.0 it is obsolete and will be removed in one of the next versions. To switch in the procfs build you need to run "make enable_proc" command before building anything else. It contains the following entries. - "help" file, which provides online help for SCST commands - "scsi_tgt" file, which on read provides information of serving by SCST devices and their dev handlers. On write it supports the following command: * "assign H:C:I:L HANDLER_NAME" assigns dev handler "HANDLER_NAME" on device with host:channel:id:lun. The recommended way to find out H:C:I:L numbers is use of lsscsi utility. - "sessions" file, which lists currently connected initiators (open sessions) - "sgv" file provides some statistic about with which block sizes commands from remote initiators come and how effective sgv_pool in serving those allocations from the cache, i.e. without memory allocations requests to the kernel. "Size" - is the commands data size upper rounded to power of 2, "Hit" - how many there are allocations from the cache, "Total" - total number of allocations. - "threads" file, which allows to read and set number of SCST's threads - "version" file, which shows version of SCST - "trace_level" file, which allows to read and set trace (logging) level for SCST. Also this file allows to dump persistent reservations information about some device in the log file. See /proc/scsi_tgt/help file for list of commands and trace levels. See also section "Dealing with massive logs" for more info how to make correct logs when you enabled trace levels producing a lot of logs data. Each dev handler has own subdirectory. Most dev handler have only two files in this subdirectory: "trace_level" and "type". The first one is similar to main SCST "trace_level" file, the latter one shows SCSI type number of this handler as well as some text description. For example, "echo "assign 1:0:1:0 dev_disk" >/proc/scsi_tgt/scsi_tgt" will assign device handler "dev_disk" to real device sitting on host 1, channel 0, ID 1, LUN 0. Access and devices visibility management (LUN masking) - /proc interface ------------------------------------------------------------------------ Access and devices visibility management allows for an initiator or group of initiators to see different devices with different LUNs with necessary access permissions. SCST supports two modes of access control: 1. Target-oriented. In this mode you define for each target devices and their LUNs, which are accessible to all initiators, connected to that target. This is a regular access control mode, which people usually mean thinking about access control in general. For instance, in IET this is the only supported mode. In this mode you should create a security group with name "Default_TARGET_NAME", where "TARGET_NAME" is name of the target, like "Default_iqn.2007-05.com.example:storage.disk1.sys1.xyz" for target "iqn.2007-05.com.example:storage.disk1.sys1.xyz". Then you should add to it all LUNs, available from that target. 2. Initiator-oriented. In this mode you define which devices and their LUNs are accessible for each initiator. In this mode you should create for each set of one or more initiators, which should access to the same set of devices with the same LUNs, a separate security group, then add to it available devices and names of allowed initiator(s). Both modes can be used simultaneously. In this case initiator-oriented mode has higher priority, than target-oriented. When a target driver registers itself in SCST core, it tells SCST core its name. Then, when there is a new connection from a remote initiator, the target driver registers this connection in SCST core and tells it the name of the remote initiator. Then SCST core finds the corresponding devices for it using the following algorithm: 1. It searches through all defined groups trying to find group containing the initiator name. If it succeeds, the found group is used. 2. Otherwise, it searches through all groups trying to find group with name "Default_TARGET_NAME". If it succeeds, the found group is used. 3. Otherwise, the group with name "Default" is used. This group is always defined, but empty by default. Names of both target and initiator you can clarify in the kernel log. In it SCST reports to which group each session is assigned. In /proc/scsi_tgt each group represented as "groups/GROUP_NAME/" subdirectory. In it there are files "devices" and "names". File "devices" lists devices and their LUNs in the group, file "names" lists names of initiators, which allowed to access devices in this group. To configure access and devices visibility management SCST provides the following files and directories under /proc/scsi_tgt: - "add_group GROUP_NAME" to /proc/scsi_tgt/scsi_tgt adds group "GROUP_NAME" - "del_group GROUP_NAME" to /proc/scsi_tgt/scsi_tgt deletes group "GROUP_NAME" - "rename_group OLD_NAME NEW_NAME" to /proc/scsi_tgt/scsi_tgt renames group "OLD_NAME" to "NEW_NAME". - "add H:C:I:L lun [READ_ONLY]" to /proc/scsi_tgt/groups/GROUP_NAME/devices adds device with host:channel:id:lun with LUN "lun" in group "GROUP_NAME". Optionally, the device could be marked as read only. The recommended way to find out H:C:I:L numbers is use of lsscsi utility. - "replace H:C:I:L lun [READ_ONLY]" to /proc/scsi_tgt/groups/GROUP_NAME/devices replaces by device with host:channel:id:lun existing with LUN "lun" device in group "GROUP_NAME" with generation of INQUIRY DATA HAS CHANGED Unit Attention. If the old device doesn't exist, this command acts as the "add" command. Optionally, the device could be marked as read only. The recommended way to find out H:C:I:L numbers is use of lsscsi utility. - "del H:C:I:L" to /proc/scsi_tgt/groups/GROUP_NAME/devices deletes device with host:channel:id:lun from group "GROUP_NAME". The recommended way to find out H:C:I:L numbers is use of lsscsi utility. - "add V_NAME lun [READ_ONLY]" to /proc/scsi_tgt/groups/GROUP_NAME/devices adds device with virtual name "V_NAME" with LUN "lun" in group "GROUP_NAME". Optionally, the device could be marked as read only. - "replace V_NAME lun [READ_ONLY]" to /proc/scsi_tgt/groups/GROUP_NAME/devices replaces by device with virtual name "V_NAME" existing with LUN "lun" device in group "GROUP_NAME" with generation of INQUIRY DATA HAS CHANGED Unit Attention. If the old device doesn't exist, this command acts as the "add" command. Optionally, the device could be marked as read only. - "del V_NAME" to /proc/scsi_tgt/groups/GROUP_NAME/devices deletes device with virtual name "V_NAME" from group "GROUP_NAME" - "clear" to /proc/scsi_tgt/groups/GROUP_NAME/devices clears the list of devices for group "GROUP_NAME" - "add NAME" to /proc/scsi_tgt/groups/GROUP_NAME/names adds name "NAME" to group "GROUP_NAME". For NAME you can use simple DOS-type patterns, containing '*' and '?' symbols. '*' means match all any symbols, '?' means match only any single symbol. For instance, "blah.xxx" will match "bl?h.*". Additionally, you can use negative sign '!' to revert the value of the pattern. For instance, "ah.xxx" will match "!bl?h.*". - "del NAME" to /proc/scsi_tgt/groups/GROUP_NAME/names deletes name "NAME" from group "GROUP_NAME" - "move NAME NEW_GROUP_NAME" to /proc/scsi_tgt/groups/OLD_GROUP_NAME/names moves name "NAME" from group "OLD_GROUP_NAME" to group "NEW_GROUP_NAME". - "clear" to /proc/scsi_tgt/groups/GROUP_NAME/names clears the list of names for group "GROUP_NAME" Examples: - "echo "add 1:0:1:0 0" >/proc/scsi_tgt/groups/Default/devices" will add real device sitting on host 1, channel 0, ID 1, LUN 0 to "Default" group with LUN 0. - "echo "add disk1 1" >/proc/scsi_tgt/groups/Default/devices" will add virtual VDISK device with name "disk1" to "Default" group with LUN 1. - "echo "21:*:e0:?b:83:*'" >/proc/scsi_tgt/groups/LAB1/names" will add a pattern, which matches WWNs of Fibre Channel ports from LAB1. Consider you need to have an iSCSI target with name "iqn.2007-05.com.example:storage.disk1.sys1.xyz" (you defined it in iscsi-scst.conf), which should export virtual device "dev1" with LUN 0 and virtual device "dev2" with LUN 1, but initiator with name "iqn.2007-05.com.example:storage.disk1.spec_ini.xyz" should see only virtual device "dev2" with LUN 0. To achieve that you should do the following commands: # echo "add_group Default_iqn.2007-05.com.example:storage.disk1.sys1.xyz" >/proc/scsi_tgt/scsi_tgt # echo "add dev1 0" >/proc/scsi_tgt/groups/Default_iqn.2007-05.com.example:storage.disk1.sys1.xyz/devices # echo "add dev2 1" >/proc/scsi_tgt/groups/Default_iqn.2007-05.com.example:storage.disk1.sys1.xyz/devices # echo "add_group spec_ini" >/proc/scsi_tgt/scsi_tgt # echo "add iqn.2007-05.com.example:storage.disk1.spec_ini.xyz" >/proc/scsi_tgt/groups/spec_ini/names # echo "add dev2 0" >/proc/scsi_tgt/groups/spec_ini/devices It is highly recommended to use scstadmin utility instead of described in this section low level interface. IMPORTANT ========= There must be LUN 0 in each security group, i.e. LUs numeration must not start from, e.g., 1. Otherwise you will see no devices on remote initiators and SCST core will write into the kernel log message: "tgt_dev for LUN 0 not found, command to unexisting LU?" IMPORTANT ========= All the access control must be fully configured BEFORE load of the corresponding target driver! When you load a target driver or enable target mode in it, as for qla2x00t driver, it will immediately start accepting new connections, hence creating new sessions, and those new sessions will be assigned to security groups according to the *currently* configured access control settings. For instance, to "Default" group, instead of "HOST004" as you may need, because "HOST004" doesn't exist yet. So, one must configure all the security groups before new connections from the initiators are created, i.e. before target drivers loaded. Access controls can be altered after the target driver loaded as long as the target session doesn't yet exist. And even in the case of the session already existing, changes are still possible, but won't be reflected on the initiator side. So, the safest choice is to configure all the access control before any target driver load and then only add new devices to new groups for new initiators or add new devices to old groups, but not altering existing LUNs in them. VDISK /proc interface --------------------- This interface starting from version 2.0.0 is obsolete and will be removed in one of the next versions. To switch to it you should run "make enable_proc". After loading VDISK device handler creates in /proc/scsi_tgt/ subdirectories "vdisk" and "vcdrom". They have the following layout: - "trace_level" and "type" files as described above - "help" file, which provides online help for VDISK commands - "vdisk"/"vcdrom" files, which on read provides information of currently open device files. On write it supports the following command: * "open NAME [PATH] [BLOCK_SIZE] [FLAGS]" - opens file "PATH" as device "NAME" with block size "BLOCK_SIZE" bytes with flags "FLAGS". "PATH" could be empty only for VDISK CDROM. "BLOCK_SIZE" and "FLAGS" are valid only for disk VDISK. The block size must be power of 2 and >= 512 bytes. Default is 512. Possible flags: - WRITE_THROUGH - write back caching disabled. Note, this option has sense only if you also *manually* disable write-back cache in *all* your backstorage devices and make sure it's actually disabled, since many devices are known to lie about this mode to get better benchmark results. - READ_ONLY - read only - O_DIRECT - both read and write caching disabled. This mode isn't currently fully implemented, you should use user space fileio_tgt program in O_DIRECT mode instead (see below). - NULLIO - in this mode no real IO will be done, but success will be returned. Intended to be used for performance measurements at the same way as "*_perf" handlers. - NV_CACHE - enables "non-volatile cache" mode. In this mode it is assumed that the target has a GOOD UPS with ability to cleanly shutdown target in case of power failure and it is software/hardware bugs free, i.e. all data from the target's cache are guaranteed sooner or later to go to the media. Hence all data synchronization with media operations, like SYNCHRONIZE_CACHE, are ignored in order to bring more performance. Also in this mode target reports to initiators that the corresponding device has write-through cache to disable all write-back cache workarounds used by initiators. Use with extreme caution, since in this mode after a crash of the target journaled file systems don't guarantee the consistency after journal recovery, therefore manual fsck MUST be ran. Note, that since usually the journal barrier protection (see "IMPORTANT" note below) turned off, enabling NV_CACHE could change nothing from data protection point of view, since no data synchronization with media operations will go from the initiator. This option overrides WRITE_THROUGH. - BLOCKIO - enables block mode, which will perform direct block IO with a block device, bypassing page-cache for all operations. This mode works ideally with high-end storage HBAs and for applications that either do not need caching between application and disk or need the large block throughput. See also below. - REMOVABLE - with this flag set the device is reported to remote initiators as removable. * "close NAME" - closes device "NAME". * "resync_size NAME" - refreshes size of device "NAME". Intended to be used after device resize. * "change NAME [PATH]" - changes a virtual CD in the VDISK CDROM. * "set_t10_dev_id NAME T10_DEVICE_ID" - sets T10 vendor specific identifier on Device Identification VPD page (0x83) of device "NAME" in INQUIRY data. By default VDISK handler always generates T10_DEVICE_ID for every new created device at creation time. This parameter allows to overwrite generated by VDISK value of T10_DEVICE_ID. By default, if neither BLOCKIO, nor NULLIO option is supplied, FILEIO mode is used. For example, "echo "open disk1 /vdisks/disk1" >/proc/scsi_tgt/vdisk/vdisk" will open file /vdisks/disk1 as virtual FILEIO disk with name "disk1". Credits ------- Thanks to: * Mark Buechler for a lot of useful suggestions, bug reports and help in debugging. * Ming Zhang for fixes and comments. * Nathaniel Clark for fixes and comments. * Calvin Morrow for testing and useful suggestions. * Hu Gang for the original version of the LSI target driver. * Erik Habbinga for fixes and support of the LSI target driver. * Ross S. W. Walker for BLOCKIO inspiration and Vu Pham who implemented it for VDISK dev handler. * Alessandro Premoli for fixes * Terry Greeniaus for fixes. * Krzysztof Blaszkowski for many fixes and bug reports. * Jianxi Chen for fixing problem with devices >2TB in size * Bart Van Assche for a lot of help * University of New Hampshire Interoperability Labs (UNH IOL, http://www.iol.unh.edu) for UNH-iSCSI project (http://www.iol.unh.edu/consortiums/iscsi/index.html) on which interface between SCST core and target drivers was based. * Daniel Debonzi for a big part of the initial SCST sysfs tree implementation Vladislav Bolkhovitin , http://scst.sourceforge.net