SCSI RDMA Protocol (SRP) Target driver for Linux
=================================================

The SRP target driver has been designed to work on top of the Linux RDMA
kernel drivers -- either the RDMA drivers included with a Linux distribution
or the OFED RDMA 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
------------

Building and installing the SRP target driver is possible as follows:

   cd ${SCST_DIR}
   if type -p rpm >/dev/null; then
      make -s rpm
      sudo rpm -U rpmbuilddir/RPMS/*/*rpm scstadmin/rpmbuilddir/RPMS/*/*rpm
   else
      make -s scst_clean srpt_clean scst srpt scstadmin
      sudo make -s scst_install srpt_install scstadm_install
   fi

The ib_srpt kernel module supports the following parameters:

* rdma_cm_port (number)
  A 16-bit number that specifies the port number to be registered via the
  RDMA/CM. Must be specified to make communication over RoCE or iWARP
  possible. If this parameter is zero (the default value) the SRP target
  driver does not register with the RDMA/CM.
* 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 4148, which corresponds to an sg table size of 255.
* 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 256)
  Per-channel InfiniBand send queue size. Depending on the queue depth,
  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
---------------------------------

When using RoCE or iWARP the first step is to enable support for these
protocols in the target driver by setting the rdma_cm_port kernel module
parameter to a non-zero value. An example:

  echo options ib_srpt rdma_cm_port=5000 > /etc/modprobe.d/ib_srpt.conf

Next, 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

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

Note: when using InfiniBand loading the ib_ipoib kernel module and assigning
an IP address to each IPoIB interface is only needed when using the RDMA/CM.
When using the IB/CM however, it is allowed but not necessary to load the
ib_ipoib kernel module.


Configuring the SRP Initiator System
------------------------------------

First of all, load the SRP kernel module as follows:

   modprobe ib_srp

Next, when using InfiniBand, discover the new SRP target by running the
srp_daemon command:

   for d in /dev/infiniband/umad*; do srp_daemon -oacd$d; done

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 (-e = execute):

   for d in /dev/infiniband/umad*; do srp_daemon -oecd$d; 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.

When using RoCE or iWARP, log in to the target system to determine the id_ext
and ioc_guid parameters and use these to log in. An example:

    [ target system ]
    # sed 's/tid_ext=/id_ext=/;s/,\(pkey\|dgid\|service_id\)=[^,]*//g' $(find /sys/kernel/scst_tgt/targets/ib_srpt -name login_info) | uniq
    id_ext=0002c90300a34270,ioc_guid=0002c90300a34270

    [ initiator system ]
    echo dest=192.168.5.1:5000,id_ext=0002c90300a34270,ioc_guid=0002c90300a34270
    >/sys/class/infiniband_srp/srp-mlx4_0-1/add_target
    echo dest=192.168.6.1:5000,id_ext=0002c90300a34270,ioc_guid=0002c90300a34270
    >/sys/class/infiniband_srp/srp-mlx4_0-2/add_target

Initiator port GIDs can be queried e.g. via sysfs:

$ for f in /sys/devices/*/*/*/infiniband/*/ports/*/gids/0; do echo $f; \
cat $f | sed 's/://g'; done
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/1/gids/0
fe800000000000000002c9030005f34b
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/2/gids/0
fe800000000000000002c9030005f34c
/sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/1/gids/0
fe800000000000000002c9030003cca7
/sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/2/gids/0
fe800000000000000002c9030003cca8

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 either
ib_srpt_target_<n>, the node GUID of a HCA in hexadecimal form with a colon
after every fourth digit or the port GID with a colon afer every fourth
digit. The HCA node GUID and the port GIDs can be obtained via the
ibv_devinfo command. An example:

# ibv_devinfo -v | grep -E '[^a-z]port:|guid|GID'
node_guid:      0002:c903:0005:f34e
sys_image_guid: 0002:c903:0005:f351
  port: 1
    GID[0]:     fe80:0000:0000:0000:0002:c903:0005:f34f
  port: 2
    GID[0]:     fe80:0000:0000:0000:0002:c903:0005:f350

Once the ib_srpt driver has been loaded the available SCST targets can be
queried as follows:

# (cd /sys/kernel/scst_tgt/targets/ib_srpt && ls -d [0-9a-f]*)
fe80:0000:0000:0000:0002:c903:0005:f34f
fe80:0000:0000:0000:0002:c903:0005:f350


Session names
-------------

The ib_srpt target driver uses the source port GID as session name.

An example:

[ INITIATOR ]

$ for f in /sys/devices/*/*/*/infiniband/*/ports/*/gids/0; do echo
f; cat $f; done
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/1/gids/0
fe80:0000:0000:0000:0002:c903:0005:f34b
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/infiniband/mlx4_0/ports/2/gids/0
fe80:0000:0000:0000:0002:c903:0005:f34c
/sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/1/gids/0
fe80:0000:0000:0000:0002:c903:0003:cca7
/sys/devices/pci0000:00/0000:00:1c.0/0000:05:00.0/infiniband/mlx4_1/ports/2/gids/0
fe80:0000:0000:0000:0002:c903:0003:cca8

[ TARGET, after login ]

$ (cd /sys/kernel/scst_tgt/targets/ib_srpt/[0-9a-f]* && ls -d sessions/*)
sessions/fe80:0000:0000:0000:0002:c903:0003:cca7
sessions/fe80:0000:0000:0000:0002:c903:0003:cca8
sessions/fe80:0000:0000:0000:0002:c903:0005:f34b
sessions/fe80:0000:0000:0000:0002:c903:0005:f34c


LUN masking
-----------

In a straightforward configuration every LUN is visible to every initiator.
It is possible however to make a different set of LUNs visible to each
initiator by using the LUN masking feature of SCST. SRP initiators are
identified by their session name (see above). An example of an scst.conf
file using LUN masking for ib_srpt:

TARGET_DRIVER ib_srpt {
        TARGET fe80:0000:0000:0000:0002:c903:0005:f34b {
                enabled 1
                rel_tgt_id 1

                # LUNs visible by all initiators not listed below
                LUN 0 disk01

                GROUP grp1 {
                        # LUNs visible by initiator system 1
                        LUN 0 disk02

                        INITIATOR fe80:0000:0000:0000:0002:c903:0005:f34b
                }

                GROUP grp2 {
                        # LUNs visible by initiator system 2
                        LUN 0 disk03

                        INITIATOR fe80:0000:0000:0000:0002:c903:0005:f34c
                }
        }
}


Adding and Removing LUNs Dynamically
------------------------------------

It is possible to add and/or remove LUNs on the target without restarting
target or initiator. This can be done either via scstadmin or directly via the
sysfs interface. Although the SCST core will notify the initiator about LUN
changes, Linux initiators will ignore these notifications. In order to bring a
Linux initiator again in sync after a LUN change, the initiator has to be told
to rescan SCSI devices. Rescanning SCSI devices is e.g. possible via the
rescsan-scsi-bus.sh script that can be found here:
http://www.garloff.de/kurt/linux/#rescan-scsi. An example:
$ rescan-scsi-bus --hosts=${srp_host_id} --channels=0 --ids=0 --luns=0-31


InfiniBand Partitions
---------------------

Just like a VLAN allows to segment traffic on an Ethernet network partitions
allow to segment traffic on an InfiniBand network. Each InfiniBand partition
is identified by a partition key which is a 16-bit number. During fabric
initialization the subnet manager assigns one or more partition keys to
each InfiniBand port. For opensm partitions are defined in
/etc/opensm/partitions.conf. ib_srpt uses the partition with index 0. Which
partition key corresponds to index 0 can be found out by querying sysfs:

$ head /sys/class/infiniband/*/ports/*/pkeys/0
==> /sys/class/infiniband/mlx4_0/ports/1/pkeys/0 <==
0xffff

==> /sys/class/infiniband/mlx4_0/ports/2/pkeys/0 <==
0xffff


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


Performance Notes - Target Side
-------------------------------

* Building the SCST core and the ib_srpt target driver in release mode
  improves performance compared to debug mode.

* When using high-latency storage devices (hard disks), the default value
  chosen by SCST for DEVICE.threads_num should be fine. When using
  low-latency storage devices though (SSDs), DEVICE.threads_num should be set
  to 1 or 2 in /etc/scst.conf in order to reach optimal performance for small
  block sizes (e.g. 4 KB).

* When multiple InfiniBand HCA's are present in a target system the Linux
  kernel by default will assign the associated interrupt handlers to CPU 0.
  Even irqbalance will often assign the interrupt handlers of multiple HCA's
  to the same CPU. That is unfortunate because it leads to unfair handling of
  SRP sessions. The solution is to assign InfiniBand HCA interrupts manually
  to different CPU's. That's possible by writing looking up the InfiniBand
  interrupt numbers in /proc/interrupts and by writing proper bitmasks into
  /proc/irq/<n>/smp_affinity.


Performance Notes - Initiator Side
----------------------------------

* Using multiple RDMA connections between initiator and target results in a
  significant performance improvement. To benefit from this feature, use
  kernel 3.19 or later at the initiator side and enable scsi-mq either by
  setting SCSI_MQ_DEFAULT=y in the kernel config or via the following command:

  echo Y > /sys/module/scsi_mod/parameters/use_blk_mq

  If the HCA model in your initiator system supports multiple MSI-X interrupts
  the next step is either to stop the irqbalance service or to write a policy
  script that stops irqbalance from modifying the IB interrupt CPU
  affinity.

  For more information about scsi-mq see also Michael Larabel, SCSI
  Multi-Queue Performance Appears Great For Linux 3.17, Phoronix, June 18,
  2014 (http://www.phoronix.com/scan.php?page=news_item&px=MTcyMjk).

* Choose a proper value for the ib_srp kernel module parameter
  cmd_sg_entries. The default value 12 works well for buffered reads while
  the throughput for write-dominated workloads improves by changing this value
  into 255. One way to set this kernel module parameter is as follows:

  echo options ib_srp cmd_sg_entries=255 >/etc/modprobe.d/ib_srp.conf

* For multithreaded workloads using small block sizes changing rq_affinity
  into 2 improves IOPS significantly (Linux kernel 3.1 and later; see also
  commit 5757a6d76cdf6dda2a492c09b985c015e86779b1).

* For latency sensitive applications, using the noop scheduler at the initiator
  side can give significantly better results than with other schedulers.

* The SRP initiator limits by default the queue depth to 64 commands. If your
  workload benefits from a larger queue depth, enlarge the queue depth by
  setting the max_cmd_per_lun and queue_size parameters in the SRP login
  string.

* 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


Performance Notes - Both Sides
------------------------------

* Disabling CONFIG_SCHED_DEBUG and CONFIG_SCHEDSTATS in the kernel config
  improves performance.

* Disable CONFIG_IRQSOFF_TRACER such that CONFIG_TRACE_IRQFLAGS is disabled.

* Consider which memory allocator to use. With recent kernels using the SLUB
  memory allocator instead of SLAB may help. On multi-socket systems the SLAB
  memory allocator may result in better performance. Please note that SLAB is
  tunable while SLUB is not. See also http://lkml.org/lkml/2010/7/9/264 and
  http://www.ibm.com/developerworks/linux/library/l-linux-slab-allocator/.


Frequently Asked Questions
--------------------------

Q: Every now and then "SRP abort called" and "SRP reset_device called"
   messages are logged at the initiator side. Around the same time I see the
   following message in the target log: "ib_srpt: ***ERROR***: Command ...: IB
   completion for idx ... has not been received in time (SRPT command state
   ...)". What is the meaning of these messages mean and how can I fix this ?

A: This means that a timeout occurred while a HCA was waiting for an
   acknowledge message. Check the IB network for bad IB cables, bad HCA's
   and/or bad switch ports. Also make sure that the HCA firmware is up to
   date.

Q: Loading the kernel module ib_srpt triggers a kernel panic with a call trace
   like the one below. What is the cause of this and how can this be solved ?

   Call Trace:
    [<ffffffffa02f2a50>] srpt_alloc_ioctx+0x60/0xb0 [ib_srpt]
    [<ffffffffa02f2f0a>] srpt_alloc_ioctx_ring+0xea/0x1e0 [ib_srpt]
    [<ffffffffa02f32e9>] srpt_add_one+0x2e9/0x670 [ib_srpt]
    [<ffffffffa015a480>] ib_register_client+0x80/0xa0 [ib_core]
    [<ffffffffa02421eb>] srpt_init_module+0x1eb/0x235 [ib_srpt]
    [<ffffffff81000344>] do_one_initcall+0x34/0x1a0
    [<ffffffff8107a63c>] sys_init_module+0xdc/0x260
    [<ffffffff81002e3b>] system_call_fastpath+0x16/0x1b

A: This means that you are using a system on which OFED has been installed but
   that ib_srpt has been compiled against the in-tree kernel headers instead
   of the OFED kernel headers. You can fix this by rebuilding ib_srpt against
   the OFED kernel headers. The ib_srpt makefile should detect the OFED kernel
   headers automatically - at least if ib_srpt is built after OFED has been
   installed.


Feedback
--------

Send questions about this driver to scst-devel@lists.sourceforge.net.