SCSI RDMA Protocol (SRP) Target driver for Linux ================================================= The SRP target driver has been designed to work on top of the Linux InfiniBand kernel drivers -- either the InfiniBand drivers included with a Linux distribution of the OFED InfiniBand drivers. For more information about using the SRP target driver in combination with OFED, see also README.ofed. The SRP target driver has been implemented as an SCST driver. This makes it possible to support a lot of I/O modes on real and virtual devices. A few examples of supported device handlers are: 1. scst_disk. This device handler implements transparent pass-through of SCSI commands and allows SRP to access and to export real SCSI devices, i.e. disks, hardware RAID volumes, tape libraries as SRP LUNs. 2. scst_vdisk, either in fileio or in blockio mode. This device handler allows to export software RAID volumes, LVM volumes, IDE disks, and normal files as SRP LUNs. 3. nullio. The nullio device handler allows to measure the performance of the SRP target implementation without performing any actual I/O. Installation ------------ Proceed as follows to compile and install the SRP target driver: 1. The SRP initiator (ib_srp) included with Linux kernel 2.6.36 and before frequently makes ib_srpt send BUSY responses, which hurts performance. This can be avoided by making SCST's SCSI command queue size identical to that of the initiator by applying the scst_increase_max_tgt_cmds patch: cd ${SCST_DIR} patch -p0 < srpt/patches/scst_increase_max_tgt_cmds.patch This patch increases SCST's per-device queue size from 48 to 64. This helps to avoid BUSY conditions because the size of the transmit queue in Linux' SRP initiator is also 64. Note: avoiding BUSY conditions is also possible by limiting the number of outstanding requests on the initiator. This is possible either by setting nr_requests low enough or by enabling the dynamic queue depth adjustment feature. Dynamic queue depth adjustment is available from kernel version 2.6.33 on. See also scst/README for more information. 2. Now compile and install SRPT: cd ${SCST_DIR} make -s scst_clean scst scst_install make -s srpt_clean srpt srpt_install make -s scstadm scstadm_install 3. Edit the installed file /etc/init.d/scst and add ib_srpt to the SCST_MODULES variable. 4. Configure SCST such that it will be started during system boot: chkconfig scst on The ib_srpt kernel module supports the following parameters: * srp_max_req_size (number) Maximum size of an SRP control message in bytes. Examples of SRP control messages are: login request, logout request, data transfer request, ... The larger this parameter, the more scatter/gather list elements can be sent at once. Use the following formula to compute an appropriate value for this parameter: 68 + 16 * (sg_tablesize). The default value of this parameter is 2116, which corresponds to an sg table size of 128. * srp_max_rsp_size (number) Maximum size of an SRP response message in bytes. Sense data is sent back via these messages towards the initiator. The default size is 256 bytes. With this value there remains (256-36) = 220 bytes for sense data. * srp_max_rdma_size (number) Maximum number of bytes that may be transferred at once via RDMA. Defaults to 65536 bytes, which is sufficient to use the full bandwidth of low-latency HCAs. Increasing this value may decrease latency for applications transferring large amounts of data at once. * srpt_srq_size (number, default 4095) ib_srpt uses a shared receive queue (SRQ) for processing incoming SRP requests. This number may have to be increased when a large number of initiator systems is accessing a single SRP target system. * srpt_sq_size (number, default 4096) Per-channel InfiniBand send queue size. The default setting is sufficient for a credit limit of 128. Changing this parameter to a smaller value may cause RDMA requests to be retried and hence may slow down data transfer severely. * trace_flag (unsigned integer, only available in debug builds) The individual bits of the trace_flag parameter define which categories of trace messages should be sent to the kernel log and which ones not. Configuring the SRP Target System --------------------------------- First of all, create the file /etc/scst.conf. You can create this file with the scstadmin tool as follows: /etc/init.d/scst stop /etc/init.d/scst start Now configure SCST using scstadmin - see also the scstadmin documentation for further information. Once finished, save the configuration to /etc/scst.conf: scstadmin -write_config /etc/scst.conf (sysfs version) or scstadmin -WriteConfig /etc/scst.conf (procfs version) One can verify the contents of scst.conf e.g. as follows: cat /etc/scst.conf Now verify that loading the configuration from file works correctly: /etc/init.d/scst reload Configuring the SRP Initiator System ------------------------------------ First of all, load the SRP kernel module as follows: modprobe ib_srp Next, discover the new SRP target by running the ibsrpdm command: ibsrpdm -c If you want to let the initiator system log in to all SRP targets available in the same InfiniBand subnet that is possible as follows: ibsrpdm -c | while read target_info; do echo "${target_info}" > /sys/class/infiniband_srp/${SRP_HCA_NAME}/add_target; done If you want to let the initiator log in to a specific target you can do that e.g. as follows: echo "id_ext=0002c903000f1366,ioc_guid=0002c903000f1366,dgid=fe800000000000000002c903000f1367,pkey=ffff,service_id=0002c903000f1366" > /sys/class/infiniband_srp/${SRP_HCA_NAME}/add_target; done The meaning of the parameters in the above command is as follows: * id_ext: must match ioc_guid. * ioc_guid: see also the documentation of the ib_srpt ioc_guid parameter. * dgid: target HCA port GID to connect to. * pkey: IB partition key (P_Key) of the target to connect to. * service_id: must match ioc_guid. Target GIDs can be queried e.g. via sysfs: $ for f in /sys/devices/*/*/*/infiniband/*/ports/*/gids/0; do echo $f; \ cat $f | cut -c21- | sed 's/://g'; done /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/1/gids/0 0002c9030005f34b /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/2/gids/0 0002c9030005f34c /sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/1/gids/0 0002c9030003cca7 /sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/2/gids/0 0002c9030003cca8 Finally run lsscsi to display the details of the newly discovered SCSI disks: lsscsi SRP targets can be recognized in the output of lsscsi by looking for the disk names assigned on the SCST target ("disk01" in the example below): [8:0:0:0] disk SCST_FIO disk01 102 /dev/sdb Target names ------------ The name assigned by the ib_srpt target driver to an SCST target is the node GUID of a HCA in hexadecimal form with a colon after every fourth digit. The HCA node GUIDs can be obtained via the ibv_devices command, the ibv_devinfo command or via sysfs. An example: # ibv_devices device node GUID ------ ---------------- mlx4_1 0002c9030003cca2 mlx4_0 0002c9030005f34e # head /sys/devices/*/*/*/infiniband/*/node_guid ==> /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/node_guid <== 0002:c903:0005:f34e ==> /sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/node_guid <== 0002:c903:0003:cca2 Once the ib_srpt driver has been loaded there will be SCST targets available with the HCA node GUID as name: # ls /sys/bus/scst_target/drivers/ib_srpt/0* 0002:c903:0003:cca2 0002:c903:0005:f34e If you need deprecated target names in form ib_srpt_target_X, you should set use_node_guid_in_target_name parameter of module ib_srpt to 0. To move from the deprecated ib_srpt_target_X layout you should replace in your scst.conf all ib_srpt_target_X to the corresponding ports names, like ib_srpt_target_0 to 0002:c902:0022:16f4, if ibv_devices reported for this port/node GUID 0002c902002216f4. Session names ------------- The ib_srpt target driver uses the 128-bit SRP initiator port identifier for the session name. This identifier is sent by the SRP initiator to the SRP target via the SRP_LOGIN_REQ information unit. The Linux SRP initiator (ib_srp) generates the initiator port identifier as follows: - The first eight bytes are the identifier extension ('initiator_ext' parameter specified in the login string echoed into the sysfs file 'add_target'). - The last eight bytes are the GUID of the initiator HCA port used to communicate with the target. An example: [ INITIATOR ] $ for f in /sys/devices/*/*/*/infiniband/*/ports/*/gids/0; do echo f; cat $f | cut -c21-; done /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/1/gids/0 0002:c903:0005:f34b /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/2/gids/0 0002:c903:0005:f34c /sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/1/gids/0 0002:c903:0003:cca7 /sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/2/gids/0 0002:c903:0003:cca8 [ TARGET, after login ] $ ls /sys/bus/scst_target/drivers/ib_srpt/*/sessions /sys/bus/scst_target/drivers/ib_srpt/0002:c903:0003:cca2/sessions: 0x00000000000000000002c9030003cca7 /sys/bus/scst_target/drivers/ib_srpt/0002:c903:0005:f34e/sessions: 0x00000000000000000002c9030005f34b High availability ----------------- If there are redundant paths in the IB network between initiator and target, automatic path failover can be set up on the initiator as follows: * Edit /etc/infiniband/openib.conf to load the SRP driver and SRP HA daemon automatically: set SRP_LOAD=yes and SRPHA_ENABLE=yes. * To set up and use the high availability feature you need the dm-multipath driver and multipath tool. * Please refer to the OFED-1.x user manual for more detailed instructions on how to enable and how to use the HA feature. See e.g. http://www.mellanox.com/related-docs/prod_software/Mellanox_OFED%20_Linux_user_manual_1_5_1_2.pdf. A setup with automatic failover between redundant targets is possible by installing and configuring DRBD on both targets. If the initiator system supports mirroring (e.g. Linux), you can use the following approach: * Configure DRBD in Active/Active mode. * Configure the initiator(s) for mirroring between the redundant targets. If the initiator system does not support mirroring (e.g. VMware ESX), you can use the following approach: * Configure DRBD in Active/Passive mode and enable STONITH mode in the Heartbeat software. For more information, see also: * http://www.drbd.org/ * http://www.linux-ha.org/wiki/Main_Page Notes about ib_srpt ------------------- * Unloading the kernel module ib_srpt while I/O is ongoing is supported. However, it can take up to two minutes before unloading finishes. During that time CPU usage will be high. Performance Notes - Initiator Side ---------------------------------- * For latency sensitive applications, using the noop scheduler at the initiator side can give significantly better results than with other schedulers. * The following parameters have a small but measurable impact on SRP performance: * /sys/class/block/${dev}/queue/rotational * /sys/class/block/${dev}/queue/rq_affinity * /proc/irq/${ib_int_no}/smp_affinity Send questions about this driver to scst-devel@lists.sourceforge.net, CC: Vu Pham and Bart Van Assche .