Installation notes for Linux iSCSI driver

 Copyright (C) 2001, 2002, Cisco Systems, Inc.
 maintained by linux-iscsi@cisco.com

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 it under the terms of the GNU General Public License as published by
 the Free Software Foundation; either version 2 of the License, or (at
 your option) any later version.

 This program is distributed in the hope that it will be useful, but
 WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 General Public License for more details.

 See the file COPYING included with this distribution for more
 details.

===============================================================================
CONTENTS
===============================================================================

  Before Installing the iSCSI Driver
  Installing And Configuring The iSCSI Driver

===============================================================================
BEFORE INSTALLING THE iSCSI DRIVER
===============================================================================

  The iSCSI drivers, README files, and example configuration files are
  available on the Linux-iSCSI homepage at:

    http://linux-iscsi.sourceforge.com

  In addition, at the website you can get information about the availability
  of new drivers, updated drivers, driver compatibility, and other relevant
  information.


===============================================================================
INSTALLING AND CONFIGURING THE iSCSI DRIVER
===============================================================================

  This section is the starting point for installing and configuring the
  iSCSI Driver for Linux and contains the following topics:

   o  Product Overview
   o  System Requirements
   o  Installation Procedures
   o  Device Names
   o  Target Bindings
   o  Mounting Filesystems
   o  Unmounting Filesystems
   o  Dynamic target and LUN discovery
   o  /proc/scsi/iscsi
   o  Uninstalling iSCSI
   o  Kernel and driver upgrades

----------------
PRODUCT OVERVIEW
----------------

  The iSCSI Driver for Linux provides an IP host with the ability to
  access storage through an IP network. The iSCSI driver acts as an iSCSI
  protocol initiator to transport SCSI requests and responses over an IP
  network between the IP host and an iSCSI target device such as a
  Cisco SN 5400 Series system. (The iSCSI protocol is an IETF-defined
  protocol for IP storage. For more information about the iSCSI protocol,
  refer to the IETF standards for IP storage at http://www.ietf.org.)

  Architecturally, the iSCSI driver combines with the IP host TCP/IP
  stack, network drivers, and NICs to provide the same functions as a SCSI
  or Fibre Channel adapter driver with an HBA. (See Figure 1.)


  FIGURE  1  iSCSI Driver Architecture

  ====================================================================
                                   |
             IP host OS            |         Storage host OS
           and applications        |         and applications
                                   |
  ====================================================================
                                   |
              +------------+       |           +-----------+
              |   iSCSI    |       |           |           |
              |   driver   |       |           |   SCSI    |
              |            |       |           |    or     |
    +---------+------------+       |           |    FC     |
    |        TCP/IP        |       |           |  adapter  |
    +----------------------+       |           |  driver   |
    |   Network drivers    |       |           |           |
    +----------------------+       |           +-----------+
               |                   |                 |
  =============|=====================================|================
               |                   |                 |
         +-----------+             |           +-----------+
         |    NIC    |             |           |    HBA    |
         +-----------+             |           +-----------+
               |                   |                 |
  =============|=====================================|================
               |                   |                 |
              _|_                  |                 |
            _(   )_                |           +-----------+
          _(  IP   )_              |           |  SCSI or  |
         (_ network _)             |           | FC media  |
           (_     _)               |           +-----------+
             (_ _)                 |                 |
               |                   |                 |
      +------------------+         |        +------------------+
      |      iSCSI       |         |        |      Storage     |
      |      target      |         |        +------------------+
      |     interface    |         |        |  ___  ___  ___   |
      +------------------+         |        | (___)(___)(___)  |
               |                   |        | (___)(___)(___)  |
               |                   |        | (___)(___)(___)  |
      +------------------+         |        | (___)(___)(___)  |
      |     Storage      |         |        +------------------+
      +------------------+         |
      |  ___  ___  ___   |         |
      | (___)(___)(___)  |         |
      | (___)(___)(___)  |         |
      | (___)(___)(___)  |         |
      | (___)(___)(___)  |         |
      +------------------+         |
                                   |
  ====================================================================

  The iSCSI driver provides a transport for SCSI requests and responses for
  storage devices; however, instead of providing a transport for directly
  attached devices, the driver transports the SCSI requests and responses
  between the IP host and an iSCSI target system via an IP network.
  The iSCSI target system, in turn, transports SCSI requests and responses
  between it and the storage devices attached to it.

  Once the iSCSI driver is installed and started, the IP host will proceed
  with a discovery process for iSCSI storage devices as follows:

  1.  The iSCSI driver requests available iSCSI targets from the
      iSCSI target system.

  2.  The iSCSI target system sends available iSCSI target names to
      the IP host.

  3.  The IP host logs in to the iSCSI targets.

  4.  The iSCSI target system accepts the IP host login.

  5.  The IP host queries targets for device information.

  6.  Targets respond with device information.

  7.  The IP host creates a table of internal devices.

  The iSCSI Driver for Linux provides IP access to a maximum of sixteen
  remote SCSI targets.  Each target will be probed for up to 256 LUNs,
  until the Linux kernel's limit of SCSI devices has been reached.

-------------------
SYSTEM REQUIREMENTS
-------------------

  This driver requires a 2.4.x Linux kernel version of 2.4.16 or later.  
  Compilation will require the kernel header files matching the kernel
  version you wish to run the driver on.  
  
  Kernels released after June 7, 2002 may or may not work with this
  river, depending on what if any changes have been made to the kernel's 
  SCSI midlayer code.

  As of June 6, 2002 there are several issues with the Linux kernel
  code that can cause problems when using SCSI devices (including iSCSI
  devices).  Linux kernels released after this date may or may not have
  fixed these problems.

  o  Linux kernels 2.2.16 through 2.2.20 and 2.4.0 through 2.4.18 are known
     to have a problem in the SCSI error recovery process. In some cases, a
     successful device reset may be ignored and the SCSI layer will continue
     on to the later stages of the error recovery process.

     The problem occurs when multiple SCSI commands for a particular device
     are queued in the low-level SCSI driver when a device reset occurs. Even
     if the low-level driver correctly reports that all the commands for
     the device have been completed by the reset, Linux will assume only
     one command has been completed and continue the error recovery
     process. (If only one command has timed out or failed, Linux will
     correctly terminate the error recovery process following
     the device reset.)

     This action is undesirable because the later stages of error recovery
     may send other types of resets, which can affect other SCSI
     initiators using the same target or other targets on the same
     bus. It is also undesirable because there are more serious bugs
     in the later stages of the Linux SCSI error recovery process.

     The Linux iSCSI driver now attempts to avoid this problem by
     replacing the usual error recovery handler for SCSI commands
     that timeout or fail.


  o  Linux kernels 2.2.16 through 2.2.20 and 2.4.0 through 2.4.2 may take
     SCSI devices offline after Linux issues a reset as part of the
     error recovery process.  Taking a device offline causes all I/O to the
     device to fail until the HBA driver is reloaded.

     After the error recovery process does a reset, it sends a SCSI
     Test Unit Ready command to check if the the SCSI target is operational
     again. If this command returns SCSI sense data, instead of correctly
     retrying the command, Linux will treat it as a fatal error, and
     immediately take the SCSI device offline.

     The Test Unit Ready will almost always be returned with sense data
     because most targets return a deferred error in the sense data of the
     first command received after a reset. This is a way of telling
     the initiator that a reset has occurred. Therefore, the affected 
     Linux kernel versions almost always take a SCSI device offline
     after a reset occurs.

     This bug is fixed in Linux kernels 2.4.3 and later.

     The Linux iSCSI driver now attempts to avoid this problem by
     replacing the usual error recovery handler for SCSI commands
     that timeout or fail.


  o  Linux kernels 2.2.16 through 2.2.21 and 2.4.0 through 2.4.18 appear to
     have problems when SCSI commands are completed with a check condition
     containing sense data. This can result in applications receiving I/O
     errors, short reads or short writes.

     After receiving a SCSI command with sense data, the Linux SCSI
     midlayer checks if some sectors of an I/O request have been
     transferred and informs the application layer if an I/O request has
     partially completed. The SCSI midlayer then retries the request for
     any sectors that have not yet been transferred. Because of the partial
     I/O request completion, applications may receive short reads or
     writes.

     All UNIX applications should handle these conditions, but there may be
     some applications which do not.

     There are also some cases where the application receives an I/O error
     rather than a short read or write.  The exact cause of the I/O errors
     is still being investigated, but it appears to be a bug in the Linux
     kernel's SCSI layer or block device layer.


  o  Linux kernels 2.2.16 through 2.2.21 and 2.4.0 through 2.4.18 may crash
     on a NULL pointer if a SCSI device is taken offline while one of the
     Linux kernel's I/O daemons (e.g. kpiod, kflushd, etc.) is trying to do
     I/O to the SCSI device. The exact cause of this problem is still being
     investigated.

     Note that some of the other bugs in the Linux kernel's error recovery
     handling may result in a SCSI device being taken offline, thus
     triggering this bug and resulting in a Linux kernel crash.


  o  Linux kernels 2.2.16 through 2.2.21 running on uniprocessors may
     hang if a SCSI disk device node is opened while the Linux SCSI
     device structure for that node is still being initialized.

     This occurs because the sd driver which controls SCSI disks
     will loop forever waiting for a device busy flag to be cleared
     at a certain point in the open routine for the disk device.
     Since this particular loop will never yield control of the 
     processor, the process initializing the SCSI disk device is
     not allowed to run, and the initialization process can never 
     clear the device busy flag which the sd driver is constantly 
     checking.

     A similar problem exists in the SCSI generic driver in some 
     2.4 kernel versions.  The sg driver may crash on a bad 
     pointer if a /dev/sg* device is opened while it is being 
     initialized.

----------
SETTING UP
----------


  1. Update /etc/iscsi.conf to include the IP addresses for your
     iSCSI targets. A sample configuration file might include entries
     like this:

       DiscoveryAddress=192.168.10.94

     The iscsi.conf man page has a more detailed description of the
     configuration file format.  To read the man page, type:

     man iscsi.conf

  2. Manually start iSCSI services to test your configuration. Run:

       service iscsi start

     If there are problems loading the iSCSI kernel module, diagnostic
     information will be placed in /var/log/iscsi.log.

     The iSCSI initialization will report information on each detected
     device to the console or in dmesg(8) output. For example:

       Vendor: SEAGATE   Model: ST39103FC         Rev: 0002
       Type:   Direct-Access                      ANSI SCSI revision: 02
       Detected scsi disk sda at scsi0, channel 0, id 0, lun 0
       SCSI device sda: hdwr sector= 512 bytes.
                                       Sectors= 17783240 [8683 MB] [8.7 GB]
       sda: sda1
       scsi singledevice 0 0 0 1

     Normal disk commands like fdisk, mkfs, and fsck will work on the
     iSCSI devices like a local drive.

     /proc/scsi/iscsi will contain a file (the controller number) that
     contains information about the iSCSI devices.

     To manually stop the iSCSI driver enter:

       service iscsi stop

  3. List your iSCSI partitions in /etc/fstab with the _netdev option.

------------
DEVICE NAMES
------------

  Because Linux assigns SCSI device nodes dynamically whenever
  a SCSI logical unit is detected, the mapping from device nodes
  (e.g /dev/sda, /dev/sdb) to iSCSI targets and logical units may 
  vary.  

  Variations in process scheduling and network delay may result in 
  iSCSI targets being mapped to different SCSI device nodes every time
  the driver is started.  Because of this variability, configuring
  applications or operating system utilities to use the standard 
  SCSI device nodes to access iSCSI devices may result in SCSI
  commands being sent to the wrong target or logical unit.

  To provide a more reliable namespace, the iSCSI driver will scan the
  system to determine the mapping from SCSI device nodes to iSCSI
  targets, and then create a tree of directories and symbolic links
  under /dev/iscsi to make it easier to use a particular iSCSI
  target's logical units.

  Under /dev/iscsi, there will be a directory tree containing 
  subdirectories for each iSCSI bus number, each target id number 
  on the bus, and each logical unit number for each target. For
  example, the whole disk device for bus 0, target id 0, LUN 0 would
  be /dev/iscsi/bus0/target0/lun0/disk 

  In each logical unit directory there will be a symbolic link for 
  each SCSI device node that may be connected to that particular 
  logical unit.  These symbolic links are modeled after the Linux 
  devfs naming convention.

  The symbolic link 'disk' will map to the whole-disk SCSI device node
  (e.g. /dev/sda, /dev/sdb, etc). 

  The symbolic links 'part1' through 'part15' will map to each
  partition of that SCSI disk (e.g. /dev/sda1, dev/sda15, etc).
  Note that these links will exists regardless of the number
  of disk partitions.  Opening the partition devices will result 
  in an error if the partition does not actually exist on the disk.

  The symbolic link 'mt' will map to the auto-rewind SCSI tape device 
  node for this LUN (e.g. /dev/st0), if any.  Additional links for 
  'mtl', 'mtm', and 'mta' will map to the other auto-rewind devices
  (e.g. /dev/st0l, /dev/st0m, /dev/st0a), regardless of whether these
  device nodes actually exist or could be opened.

  The symbolic link 'mtn' will map to the no-rewind SCSI tape device 
  node for this LUN (e.g. /dev/nst0), if any.  Additional links for 
  'mtln', 'mtmn', and 'mtan' will map to the other no-rewind devices
  (e.g. /dev/nst0l, /dev/nst0m, /dev/nst0a), regardless of whether 
  those device nodes actually exist or could be opened.

  The symbolic link 'cd' will map to the SCSI cdrom device node
  for this LUN (e.g. /dev/scd0), if any.

  The symbolic link 'generic' will map to the SCSI generic device 
  node for this LUN (e.g. /dev/sg0), if any.

  Because the symlink creation process must open all of the SCSI
  device nodes in /dev in order to determine which nodes map to
  iSCSI devices, you may see many modprobe messages logged to syslog
  indicating that modprobe could not find a driver for a particular
  combination of major and minor numbers.  This is harmless, and can
  be ignored.  The messages occur when Linux is unable to find a
  driver to associate with a SCSI device node that the iSCSI daemon
  is opening as part of it's symlink creation process.  To prevent
  these messages, the SCSI device nodes with no associated high-level
  SCSI driver can be removed.

---------------
TARGET BINDINGS
---------------

  The iSCSI driver automatically maintains a bindings file
  /var/iscsi/bindings.  This file contains persistent bindings
  to ensure that the same iSCSI bus and target id number are
  used for every iSCSI session to a particular iSCSI TargetName,
  no matter how many times the driver is restarted.

  This feature ensures that the SCSI numbers in the device symlinks
  described above will always map to the same iSCSI target.

  Note that because of the way Linux dynamically allocates SCSI device
  nodes as SCSI devices are found, the driver does not and can not
  ensure that any particular SCSI device node (e.g. /dev/sda) will
  always map to the same iSCSI TargetName.  The symlinks described
  in the section on Device Names are intended to provide a persistent
  device mapping for use by applications and fstab files, and should
  be used instead of direct references to particular SCSI device nodes.  

  If the bindings file grows too large, lines for targets that no
  longer exist may be manually removed by editing the file.  Manual
  editing should not normally be needed, since the driver can maintain
  up to 65535 different bindings.
	
--------------------
MOUNTING FILESYSTEMS
--------------------

  Because the Linux boot process normally mounts filesystems listed in
  /etc/fstab before the network is configured, you need to add the
  _netdev option to each filesystem on an iSCSI device.

  Because of the variability of the mapping between SCSI device nodes
  and iSCSI targets, instead of directly mounting SCSI device nodes, 
  it is recommended to either mount the /dev/iscsi tree symlinks,
  mount filesystem UUIDs or labels (see man pages for mke2fs, mount, 
  and fstab), or use logical volume management (see Linux LVM) to
  avoid mounting the wrong device due to device name changes resulting
  from iSCSI target configuration changes or network delays.

----------------------
UNMOUNTING FILESYSTEMS
----------------------
	
  It is very important to unmount all filesystems on iSCSI devices
  before the iSCSI driver stops.  If the iSCSI driver stops while
  iSCSI devices are mounted, buffered writes may not be committed to
  disk and filesystem corruption may occur.

  For this reason, the system automatically umounts all file
  systems with the _netdev option before the iSCSI driver
  stops and before the network is stopped.
	
  Since Linux will not unmount filesystems that are being used by a
  running process, before iSCSI devices can be unmounted, any
  processes using those devices must be stopped (see fuser(1)).

  To avoid filesystem corruption, the netfs shutdown script will
  automatically kill all processes using _netdev devices in /etc/fstab,
  first by sending them SIGTERM, and then by sending any remaining
  processes SIGKILL. It will then unmount all iSCSI filesystems and
  kill the iSCSI daemon, terminating all connections to iSCSI devices.

--------------------------------
DYNAMIC TARGET AND LUN DISCOVERY
--------------------------------

  The driver can be told to rediscover iSCSI devices and probe for LUNs
  by running:

  service iscsi reload

  This will cause the iSCSI daemon to restart all iSCSI discovery 
  processes and probe LUNs on all iSCSI targets.

  In addition, when using iSCSI targets that support long-lived
  iSCSI discovery sessions, such as the Cisco 5400 Series, the driver
  will keep a discovery session open waiting for change notifications
  from the target.  When a notification is received, the driver will
  rediscover targets, add any new targets, and reprobe LUNs on all
  targets that were discovered.  

-----------------
/proc/scsi/iscsi
-----------------

  The directory /proc/scsi/iscsi will contain a special file that can be
  used to get status from your iSCSI HBA.  The name of the file will
  be the iSCSI HBA's host number, which is assigned to the driver
  by Linux.

  When the file is read, it will show the driver's version number,
  followed by a list all iSCSI targets and LUNs the driver has found
  and can use.  

  Each line will show the Linux host number, channel number, target id
  number, and logical unit number, as well as the IP address, TCP port,
  and iSCSI TargetName. If no LUNs have yet been found for a target, 
  the LUN number field will contain a question mark.

------------
UNINSTALLING
------------

  rpm -e iscsi

--------------------------
KERNEL AND DRIVER UPGRADES
--------------------------

  Use up2date to get newer kernels and iscsi packages when they are
  available.
