scst/srpt

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 or 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
------------

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

   cd ${SCST_DIR}
   make -s scst_clean scst scst_install
   make -s srpt_clean srpt srpt_install
   make -s scstadm scstadm_install

The ib_srpt kernel module supports the following parameters:
* one_target_per_port (boolean) and
* use_node_guid_in_target_name (boolean)
  ib_srpt can operate in one of the following three modes:
  1. Access control configuration per HCA and assigning a "ib_srpt_target_<n>"
     style name to each HCA.
  2. Access control configuration per HCA and referring to a HCA via its node
     GUID (e.g. 0002:c903:0005:f34a).
  3. Access control configuration per HCA port and referring to a HCA via its
     port GID (e.g. fe80:0000:0000:0000:0002:c903:0005:f34b).
  Mode (1) is choosen if both one_target_per_port and
  use_node_guid_in_target_name are false. Mode (2) is choosen if
  one_target_per_port is false and use_node_guid_in_target_name is true. Mode
  (3) is choosen if one_target_per_port is true.
* 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 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
---------------------------------

The first step is to choose whether access control will be controlled per
HCA or per HCA port and to create a modprobe configuration file that reflects
this choice. An example:

  # cat /etc/modprobe.d/ib_srpt.conf
  options ib_srpt one_target_per_port=1

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  (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 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.

Target 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 GUID with a colon afer every fourth
digit. The HCA node and port GUIDs 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 name assigned by the ib_srpt target driver to a session depends on the
mode in which it is operating. If one_target_per_port=y then the source port
GID is used as the session name. If one_target_per_port=n then the 128-bit SRP
initiator port identifier is used as 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; 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:c903:0005:f34b


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
  choosen 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
----------------------------------

* 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 parameter 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
  helps.

* 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, CC:
Vu Pham <vuhuong@mellanox.com> and Bart Van Assche <bvanassche@acm.org>.