[mirror/scst/.git] / scst / README

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

Version 1.0.1, X XXXX 2008
--------------------------

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 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 virtual SCSI devices in the SCST
   environment

 * "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.

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

Only vanilla kernels from kernel.org are supported, but it should work
on vendors' kernels, if you manage to successfully compile on them. The
main problem with vendor's 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 change as necessary the corresponding to that symbol "#if
LINUX_VERSION_CODE" statement.

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 scst_exec_req_fifo.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 you don't need pass-through support or CONFIG_SCST_STRICT_SERIALIZING is
defined during the compilation (see description below).

Then, to get the maximum performance you should apply export_alloc_io_context
patch. This patch simply makes alloc_io_context() function be available
for modules, not only for built-in in kernel code.

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 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.

It is highly recommended to use scstadmin utility for configuring
devices and security groups.

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 isn't
=========  supported. This is a limitation of the Linux memory/cache                      
           manager, because in this case an OOM deadlock like: system
           needs some memory -> it decides to clear some cache -> cache
           needs to write on a target exported device -> initiator sends
           request to the target -> target needs memory -> problem is
           possible.

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 scst_uninstall'.

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 the next release. 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:

 - 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 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.

 - CONFIG_SCST_DEBUG_TM - if defined, turns on task management functions
   debugging, when on LUN 0 in the default access control 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
   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_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, no kernel patching is necessary.

 - CONFIG_SCST_ALLOW_PASSTHROUGH_IO_SUBMIT_IN_SIRQ - if defined, it will be
   allowed to submit pass-through commands to real SCSI devices via the SCSI
   middle layer using scsi_execute_async() function from soft IRQ
   context (tasklets). This used to be the default, but currently it
   seems the SCSI middle layer starts expecting only thread context on
   the IO submit path, so it is disabled now by default. Enabling it
   will decrease amount of context switches and improve performance. It
   is more or less safe, in the worst case, if in your configuration the
   SCSI middle layer really doesn't expect SIRQ context in
   scsi_execute_async() function, you will get a warning message in the
   kernel log.

 - 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 /proc/scsi_tgt/latency
   file average commands processing latency. You can clear already
   measured results by writing 0 in this file. Note, you need a
   non-preemtible kernel to have correct results.

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 it, 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 "/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. If you want to
    enable logging options, 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, I use 25,
       but to use it I needed to modify the maximum allowed value for
       CONFIG_LOG_BUF_SHIFT in the corresponding Kconfig.

     * Change in your /etc/syslog.conf or other config file of your favorite
       logging program to store kernel logs in async manner. For example,
       I added in my rsyslog.conf line "kern.info -/var/log/kernel"
       and added "kern.none" in line for /var/log/messages, so I had:
       "*.info;kern.none;mail.none;authpriv.none;cron.none /var/log/messages"

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 views of LUs/LUNs (security groups)
each with appropriate access permissions. It is highly recommended to
use scstadmin utility for that purpose instead of described in this
section low level interface.

Initiator is represented as an SCST session. The session is bound 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 bound to security group with provided
name. If no such groups found, the session bound to either
"Default_target_name", or "Default" group, depending from either
"Default_target_name" exists or not. In "Default_target_name" target
name means name of the target.

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

There must be LUN 0 in each security group, i.e. LUs numeration must not
start from, e.g., 1.

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. 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".

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

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".

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: 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). 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. Also note
           that on some real-life workloads write through caching might
           perform better, than write back one with the barrier
           protection turned on.
           Also you should realize that Linux doesn't provide a
           guarantee that after sync()/fsync() all written data really
           hit permanent storage, they can be then in the cache of your
           backstorage device and lost on power failure event. Thus,
           ever with write-through cache mode, you still need a good UPS
           to protect yourself from your data loss (note, data loss, not
           the file system integrity corruption).

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.

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
=========  them, if you try to use a device in both those modes simultaneously,
           you will almost instantly corrupt your data on that device.

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 hardware on target as is, without any
modifications. 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 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.sf.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
-----------

Before doing any performance measurements note that:

I. 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.

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

1. For SCST:

 - Disable in Makefile CONFIG_SCST_STRICT_SERIALIZING, CONFIG_SCST_EXTRACHECKS,
   CONFIG_SCST_TRACING, CONFIG_SCST_DEBUG*, CONFIG_SCST_STRICT_SECURITY

 - For pass-through devices enable
   CONFIG_SCST_ALLOW_PASSTHROUGH_IO_SUBMIT_IN_SIRQ.

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.

 - If your initiator(s) use dedicated exported from the target virtual
   SCSI devices and have more or equal amount of memory, than the
   target, it is recommended to use O_DIRECT option (currently it is
   available only with fileio_tgt user space program) or BLOCKIO. With
   them you could have up to 100% increase in throughput.

IMPORTANT: Some of the compilation options enabled by default, i.e. SCST
=========  is optimized currently rather for development and bug hunting,
           than for performance.

If you use SCST version taken directly from the SVN repository, you can
set the above options, except CONFIG_SCST_ALLOW_PASSTHROUGH_IO_SUBMIT_IN_SIRQ,
using debug2perf Makefile target.

4. For other target and initiator software parts:

 - 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. To do it on Linux initiators, 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.

 - 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 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.

5. For hardware on target.

 - Make sure that your target hardware (e.g. target FC or network card)
   and underlaying 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.

6. 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.

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.

What if target's backstorage is too slow
----------------------------------------

If under high load you experience I/O stalls or see in the kernel log on
the target abort or reset messages, then your backstorage is too slow
comparing with your target link speed and amount of simultaneously
queued commands. 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.

Unfortunately, currently SCST lacks dynamic I/O flow control, when the
queue depth on the target is dynamically decreased/increased based on
how slow/fast the backstorage speed comparing to the target link. So,
there are only 5 possible actions, which you can do to workaround or fix
this issue:

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 put offline, i.e. if the backstorage isn't
too much 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.

3. Try to avoid such seek intensive workloads.

4. Insrease speed of the target's backstorage.

5. Implement in SCST dynamic I/O flow control. See "Dynamic I/O flow
control" section on http://scst.sourceforge.net/contributing.html page
for possible idea how to do it.

To decrease device queue depth on Linux initiators 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.

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, 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.

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 useful
   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.

 * Michael G. Byrnes <michael.byrnes@hp.com> for fixes.

 * Alessandro Premoli <a.premoli@andxor.it> for fixes

 * Nathan Bullock <nbullock@yottayotta.com> for fixes.

 * Terry Greeniaus <tgreeniaus@yottayotta.com> for fixes.

 * Krzysztof Blaszkowski <kb@sysmikro.com.pl> for many fixes and bug reports.

 * Jianxi Chen <pacers@users.sourceforge.net> for fixing problem with
   devices >2TB in size

 * Bart Van Assche <bart.vanassche@gmail.com> for a lot of help

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