scst/scst

Generic SCSI target mid-level for Linux (SCST)
==============================================

Version 0.9.6, XX XXX 200X
--------------------------

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 in "Generic SCSI Target Middle Level for Linux" document
SCST's Internet page http://scst.sourceforge.net.

SCST looks to be quite stable (for beta) and useful. It supports disks
(SCSI type 0), tapes (type 1), processor (type 3), CDROM's (type 5), MO
disks (type 7), medium changers (type 8) and RAID controller (type 0xC)
as well as VDISK and "performance" device handlers. In addition, it
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.

This is quite stable (but still beta) version.

Tested mostly on "vanilla" 2.6.17.8 kernel from kernel.org.

Installation
------------

At first, make sure that the link "/lib/modules/`you_kernel_version`/build" 
points to the source code for your currently running kernel.

Then, since in the mainstream kernels scsi_do_req()/scsi_execute_async()
work in LIFO order, instead of expected and required FIFO, SCST needs a
new functions scsi_do_req_fifo()/scsi_execute_async_fifo() to be added
in the kernel. Patch 26_scst-2.6.X.patch from "kernel" directory does
that. If it doesn't apply to your kernel, apply it manually, it only
adds one of those functions and nothing more. You may not patch the
kernel if STRICT_SERIALIZING or VDISK_ONLY are defined during the
compilation (see their description below).

To compile SCST type 'make'. It will build SCST itself and its device
handlers. To install them type 'make install'. The driver modules will
be installed in '/lib/modules/`you_kernel_version`/extra'. In addition,
scsi_tgt.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 drive_name'. The names are:

 - scsi_tgt - 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 (disk or ISO CD image).

Then, to see your devices remotely, you need to add them to at least
"Default" security group (see below how). By default, no local devices
are seen remotely. There must be LUN 0 in each security group, i.e. LUs
numeration must not start from, e.g., 1.

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 "scsi add-single-device A 0 0 B" >/proc/scsi/scsi',
	   where A - is the host number, B - LUN.

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 look to
	   be safe.

To uninstall, type 'make uninstall'.

If you install QLA2x00 target driver's source code in this directory,
then you can build, install or uninstall it by typing 'make qla', 'make
qla_install' or 'make qla_uninstall' correspondingly. For more details
about QLA2x00 target drivers see their README files.

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 and
"performance" ones.

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 transfered 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 the next release. Virtual CDROM's are useful for remote
installation. See below for details how to setup and use VDISK device
handler.

"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:

 - VDISK_ONLY - if defined, the pass-through device handlers
   (scst_disk, scst_tape) will not work, but SCST will not require the
   kernel patching. Defined by default to ease new people try SCST on
   their kernels.

 - DEBUG - turns on some debugging code, including some logging. Makes
   the driver considerably bigger and slower, producing large amount of
   log data.

 - TRACING - turns on ability to log events. Makes the driver considerably
   bigger and lead to some performance loss.

 - EXTRACHECKS - adds extra validity checks in the various places.

 - DEBUG_TM - turns on task management functions debugging, when on
   LUN 0 in the "Default" group 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 set TM_DBG_GO_OFFLINE symbol in the
   Makefile to 1 if you want that the device eventually become
   completely unresponsive, or to 0 otherwise to circle around ABORTs
   and RESETs code. Needs DEBUG turned on.

 - STRICT_SERIALIZING - 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 set 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, no kernel
   patching is necessary.

 - SCST_HIGHMEM - if defined on HIGHMEM systems with 2.6 kernels, it
   allows SCST to use HIGHMEM. This is very experimental feature and it
   is unclear, if it brings something valuable, except some performance
   hit, so in the current version it is disabled. Note, that
   SCST_HIGHMEM isn't required for HIGHMEM systems and SCST will work
   fine on them with SCST_HIGHMEM off. Untested.
  
 - SCST_STRICT_SECURITY - if defined, makes SCST clean 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.

Module parameters
-----------------

Module scsi_tgt 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 "/proc" commands
---------------------

For communications with user space programs SCST provides proc-based
interface in "/proc/scsi_tgt" directory. 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

  - "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. See "help" file for list of trace levels.

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)
------------------------------------------------------

Access and devices visibility management allows for an initiator or
group of initiators to have different limited set of LUs/LUNs (security
group) each with appropriate access permissions. Initiator is
represented as a SCST session. Session is binded to security group on
its registration time by character "name" parameter of the registration
function, which provided by target driver, based on its internal
authentication. For example, for FC "name" could be WWN or just loop
ID. For iSCSI this could be iSCSI login credentials or iSCSI initiator
name. Each security group has set of names assigned to it by system
administrator. Session is binded to security group with provided name.
If no such groups found, the session binded to "Default" group.

In /proc/scsi_tgt each group represented as "groups/GROUP_NAME/"
subdirectory. In it there are files "devices" and "users". File
"devices" lists all devices and their LUNs in the group, file "users"
lists all names that should be binded to this group.

To configure access and devices visibility management SCST provides the
following files and directories under /proc/scsi_tgt:

  - "add_group GROUP" to /proc/scsi_tgt/scsi_tgt adds group "GROUP"
  
  - "del_group GROUP" to /proc/scsi_tgt/scsi_tgt deletes group "GROUP"
  
  - "add H:C:I:L lun [READ_ONLY]" to /proc/scsi_tgt/groups/GROUP/devices adds 
    device with host:channel:id:lun as LUN "lun" in group "GROUP". Optionally,
    the device could be marked as read only.
  
  - "del H:C:I:L" to /proc/scsi_tgt/groups/GROUP/devices deletes device with
    host:channel:id:lun from group "GROUP"
  
  - "add V_NAME lun [READ_ONLY]" to /proc/scsi_tgt/groups/GROUP/devices adds
    device with virtual name "V_NAME" as LUN "lun" in group "GROUP".
    Optionally, the device could be marked as read only.
  
  - "del V_NAME" to /proc/scsi_tgt/groups/GROUP/devices deletes device with
    virtual name "V_NAME" from group "GROUP"
  
  - "clear" to /proc/scsi_tgt/groups/GROUP/devices clears the list of devices
    for group "GROUP"
  
  - "add NAME" to /proc/scsi_tgt/groups/GROUP/names adds name "NAME" to group 
    "GROUP"
  
  - "del NAME" to /proc/scsi_tgt/groups/GROUP/names deletes name "NAME" from group 
    "GROUP"
  
  - "clear" to /proc/scsi_tgt/groups/GROUP/names clears the list of names
    for group "GROUP"

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. 

VDISK device handler
--------------------

After loading VDISK device handler creates in "/proc/scsi_tgt/"
subdirectories "vdisk" and "vcdrom". They have similar layout:

  - "trace_level" and "type" files as described for other dev handlers
  
  - "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
      
      - READ_ONLY - read only
      
      - O_DIRECT - both read and write caching disabled (doesn't work
        currently).

      - 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 GOOD UPS and software/hardware bug
	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 (BTW, so violating SCSI standard) in order to bring
	a bit more performance. 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. The main intent for it is to determine
	the performance impact caused by the cache synchronization.
	Note, that since usually the journal barrier protection (see
	"IMPORTANT" below) turned off, enabling NV_CACHE could change
	nothing, since no data synchronization with media operations
	will go from the initiator.

      - BLOCKIO - enables block mode, which will perform direct block
        IO with the 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.
    
    * "close NAME" - closes device "NAME".

    * "change NAME [PATH]" - changes a virtual CD in the VDISK CDROM.

For example, "echo "open disk1 /vdisks/disk1" >/proc/scsi_tgt/vdisk/vdisk"
will open file /vdisks/disk1 as virtual VDISK disk with name "disk1".

IMPORTANT: By default for performance reasons VDISK FILEIO devices use write
=========  back caching policy. This is generally safe from the consistence of
           journaled file systems, laying over them, point of view, but
	   your unsaved cached data will be lost in case of
	   power/hardware/software failure, so you must supply your
	   target server with some kind of UPS or disable write back
	   caching using WRITE_THROUGH flag. You also should note, that
	   the file systems journaling over write back caching enabled
	   devices works reliably *ONLY* if the order of journal writes
	   is guaranteed or it uses some kind of data protection
	   barriers (i.e. after writing journal data some kind of
	   synchronization with media operations is used), otherwise,
	   because of possible reordering in the cache, even after
	   successful journal rollback, you very much risk to loose your
	   data on the FS. Currently, Linux IO subsystem guarantees
	   order of write operations only using data protection
	   barriers. Some info about it from the XFS point of view could
	   be found at http://oss.sgi.com/projects/xfs/faq.html#wcache.
	   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. Note, that
	   usually it turned off by default and the status of barriers
	   usage isn't reported anywhere in the system logs as well as
	   there is no way to know it on the mounted file system (at
	   least no known one). Also note that on some real-life
	   workloads write through caching might perform better, than
	   write back one with the barrier protection turned on.

IMPORTANT: Many disk and partition table management utilities don't support
=========  block sizes >512 bytes, therefore make sure that your favorite one
           supports it. Also, if you export disk file or device with
	   some block size, different from one, with which it was
	   already divided on partitions, 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.

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/triary 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.

Performance
-----------

Before doing any performance measurements note that:

I. Currently maximum performance is possible only with real SCSI devices
or VDISK BLOCKIO mode with several simultaneously executed commands
(SCSI tagged queuing) or performance handlers. If you have enough CPU
power, VDISK FILEIO handler also could provide the same results, when
aggregate throughput is close to the aggregate throughput locally on the
target from the same disks. Also note, that currently IO subsystem in
Linux implemented on such way, so a VDISK FILEIO device over a single
file occupied entire formatted with some file system device (eg
/dev/hdc) could perform considerably better, than a VDISK FILEIO device
over /dev/hdc itself without the file system involved.

II. In order to get the maximum performance you should:

1. For SCST:

 - Disable in Makefile STRICT_SERIALIZING, EXTRACHECKS, TRACING, DEBUG*,
   SCST_STRICT_SECURITY, SCST_HIGHMEM

2. For Qlogic target driver:

 - Disable in Makefile EXTRACHECKS, TRACING, DEBUG_TGT, DEBUG_WORK_IN_THREAD

3. For device handlers, including VDISK:

 - Disable in Makefile TRACING, DEBUG

IMPORTANT: Some of those options enabled by default, i.e. SCST is optimized
=========  currently rather for development, not for performance.

4. For kernel:

 - Don't enable debug/hacking features, 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 (our case), which sometimes could lead to
   unexpectedly low throughput. You should increase read-ahead size
   (/sys/block/device/queue/read_ahead_kb) to at least 256Kb or even
   more on all initiators and the target. Also experiment with other
   parameters in /sys/block/device directory, they also affect the
   performance. If you find the best values, please share them with us.

 - Also it is recommended to turn the kernel preemption off, i.e. set
   the kernel preemption model to "No Forced Preemption (Server)".

5. For hardware.

 - Make sure that your target hardware (e.g. target FC card) and underlaying
   SCSI hardware (e.g. SCSI card to which your disks connected) stay on
   different PCI buses. They will have to work in parallel, so it
   will be better if they don't race for the bus. The problem is not
   only in the bandwidth, which they have to share, but also in the
   interaction between the cards during that competition. We have told
   that in some cases it could lead to 5-10 times less performance, than
   expected.

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 "unusual" write access
	   pattern, with which Windows'es write data and which is
	   (simplifying) incompatible with how Linux page cache works,
	   so for each write the corresponding block must be read first.
	   With 4096 bytes block sizes for VDISK devices the write
	   performance will be as expected. Actually, any system on
	   initiator, 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 on the target.

Just for reference: we had with 0.9.2 and "old" Qlogic driver on 2.4.2x
kernel, where we did careful performance study, aggregate throughput
about 390 Mb/sec from 2 qla2300 cards sitting on different 64-bit PCI
buses and working simultaneously for two different initiators with
several simultaneously working load programs on each. From one card -
about 190 Mb/sec. We used tape_perf handler, so there was no influence
from underlying SCSI hardware, i.e. we measured only SCST/FC overhead.
The target computer configuration was not very modern for the moment:
something like 2x1GHz Intel P3 Xeon CPUs. You can estimate the
memory/PCI speed from that. CPU load was ~5%, there were ~30K IRQ/sec
and no additional SCST related context switches.

Credits
-------

Thanks to:

 * Mark Buechler <mark.buechler@gmail.com> for a lot of useful
   suggestions, bug reports and help in debugging.

 * Ming Zhang <mingz@ele.uri.edu> for fixes and comments.
 
 * Nathaniel Clark <nate@misrule.us> for fixes and comments.

 * Calvin Morrow <calvin.morrow@comcast.net> for testing and usful
   suggestions.

 * Hu Gang <hugang@soulinfo.com> for the original version of the
   LSI target driver.

 * Erik Habbinga <erikhabbinga@inphase-tech.com> for fixes and support
   of the LSI target driver.

 * Ross S. W. Walker <rswwalker@hotmail.com> for the original block IO
   code and Vu Pham <huongvp@yahoo.com> who updated it for the VDISK dev
   handler.

Vladislav Bolkhovitin <vst@vlnb.net>, http://scst.sourceforge.net