Installation notes for iSCSI Driver Version for Linux Copyright (C) 2001, 2002, Cisco Systems, Inc. maintained by This program is free software; you can redistribute it and/or modify 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 =============================================================================== Product Overview Before Installing The Driver Installing And Removing The Driver Configuring And Using The Driver =============================================================================== PRODUCT OVERVIEW =============================================================================== The Cisco iSCSI Driver provides a host with the ability to access storage through an IP network. The driver uses the iSCSI protocol to transport SCSI requests and responses over an IP network between the host and a Cisco SN 5400 Series Storage Router. 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 Architecturally, the iSCSI driver combines with the host's TCP/IP stack, network drivers, and Network Interface Card (NIC) to provide the same functions as a SCSI or a Fibre Channel (FC) adapter driver with a Host Bus Adapter (HBA) (see Figure 1). Figure 1 - iSCSI Driver Architecture iSCSI Access To Storage | SCSI or FC Access to Storage ==================================================================== | Host applications | Host applications | =======|===========|===============================|================ | | | | | +------------+ | | | | SCSI | | | | | driver | | | | +------------+ | +-----------+ | | iSCSI | | | | | | driver | | | SCSI | +---------+------------+ | | or FC | | TCP/IP | | | adapter | +----------------------+ | | driver | | Network drivers | | | | +----------------------+ | +-----------+ | | | =============|=====================================|================ | | | +-----------+ | +-----------+ | NIC | | | HBA | +-----------+ | +-----------+ | | | =============|=====================================|================ _|_ | _|_ _( )_ | _( )_ _( IP )_ | _( FC )_ (_ network _) | (_ or SCSI _) (_ _) | (_ _) (_ _) | (_ _) | | | +------------------+ | | | SN 5400 Series | | | | Storage Router | | | +------------------+ | | | | | +------------------+ | +------------------+ | Storage | | | Storage | +------------------+ | +------------------+ | (___)(___)(___) | | | (___)(___)(___) | | (___)(___)(___) | | | (___)(___)(___) | | (___)(___)(___) | | | (___)(___)(___) | | (___)(___)(___) | | | (___)(___)(___) | +------------------+ | +------------------+ | ==================================================================== The iSCSI driver provides a transport for SCSI requests and responses to storage devices via an IP network instead of using a direct attached SCSI bus channel or an FC connection. The SN 5400 Series Storage Router, in turn, transports these SCSI requests and responses received via the IP network between it and the storage devices attached to it. Once the iSCSI driver is installed, the host will proceed with a discovery process for storage devices as follows: 1. The iSCSI driver requests available targets through the SendTargets discovery mechanism as configured in the /etc/iscsi.conf configuration file. 2. Each iSCSI target sends available iSCSI target names to the iSCSI driver. 3. The iSCSI driver discovery daemon process looks up each discovered target in the /etc/iscsi.bindings file. If an entry exists in the file for the target, the corresponding SCSI target ID is assigned to the target. If no entry exists for the target, the smallest available SCSI target ID is assigned and an entry is written to the /etc/iscsi.bindings file. The driver then sends a login request to the iSCSI target. 4. The iSCSI target accepts the login and sends target identifiers. 5. The iSCSI driver queries the targets for device information. 6. The targets respond with the device information. 7. The iSCSI driver creates a table of available target devices. Once the table is completed, the iSCSI targets are available for use by the host using all the same commands and utilities as a direct attached (e.g., via a SCSI bus) storage device. =============================================================================== BEFORE INSTALLING THE DRIVER =============================================================================== The iSCSI driver, README file, and example configuration files are available on the Linux-iSCSI homepage at: 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 REMOVING THE DRIVER =============================================================================== This section describes the procedures for installing and removing the Cisco iSCSI Driver covering the following topics: System Requirements Installation Procedure Removal Procedure Driver Upgrade Procedure ------------------- SYSTEM REQUIREMENTS ------------------- The Cisco iSCSI Driver for Linux requires a host running the Linux operating system with a 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. If you are running a kernel binary compiled for you by a Linux vendor, the iSCSI driver must be compiled against the source code distributed by the Linux vendor, without any modifications. The process of compiling a custom kernel from source makes modifications to the files in the kernel source tree, and an iSCSI driver compiled against a modified kernel source tree may not run on a kernel binary distributed by your Linux vendor. If you have already made changes to the kernel source tree and you wish to run the iSCSI driver on one of the vendor's kernel binaries, you will need to reinstall the kernel source code provided by your Linux vendor. Kernels released after January 11, 2003 may or may not work with this driver, depending on what if any changes have been made to the kernel's SCSI mid-layer code. 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. - All Linux kernels released on or before Feb 4, 2002 have a known bug in the buffer and page cache design. When any writes to a buffered block device fail, it is possible for the unwritten data to be discarded from the caches, even though the data was never written to disk. Any future reads will get the prior contents of the disk, and it is possible for applications to get no errors reported. This occurs because block I/O write failures from the buffer cache simply mark the buffer invalid when the write fails. This leaves the buffer marked clean and invalid, and it may be discarded from the cache at any time. Any future read either finds no existing buffer or finds the invalid buffer, so the read will fetch old data from disk and place it in the cache. If the fsync(2) function initiated the write, an error may be returned. If memory pressure on the cache initiated the write, the unwritten buffer may be discarded before fsync(2) is ever called, and in that case fsync will be unaware of the data loss, and will incorrectly report success. There is currently no reliable way for an application to ensure that data written to buffered block devices has actually been written to disk. Buffered data may be lost whenever a buffered block I/O device fails a write. The iSCSI driver attempts to avoid this problem by retrying disk commands for many types of failures. The MinDiskCommandTimeout defaults to "infinite", which disables the command timeout, allowing commands to be retried forever if the storage device is unreachable or unresponsive. - The Redhat Advanced Server 2.1 kernels released as of January 29, 2003 have a kernel bug that can cause umount to oops. See bugs and for more details. Please contact Redhat and request a kernel update if you encounter this problem. - All Linux kernels up to and including 2.4.20 have a bug in the SCSI device initialization code. If kernel memory is low, the initialization code can fail to allocate command blocks needed for proper operation, but will do nothing to prevent I/O from being queued to the non-functional device. If a process queues an I/O request to a SCSI device that has no command blocks allocated, that process will block forever in the kernel, never exiting and ignoring all signals sent to it while blocked. If the LUN probes initiated by the iSCSI driver are blocked forever by this problem, it will not be possible to stop or unload the iSCSI driver, since the driver code will still be in use. In addition, any other LUN probes initiated by the iSCSI driver will also block, since any other probes will block waiting for the probe currently in progress to finish. When the failure to allocate command blocks occurs, the kernel will log a message similar to the following: *************************************************************** kernel: scsi_build_commandblocks: want=12, space for=0 blocks In some cases, the following message will also be logged: kernel: scan_scsis: DANGER, no command blocks *************************************************************** - 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. - 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 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. - Linux kernels 2.2.16 through 2.2.21 and 2.4.0 through 2.4.20 appear to have problems when SCSI commands to disk devices are completed with a check condition/unit attention containing deferred sense data. This can result in applications receiving I/O errors, short reads or short writes. The Linux SCSI code may deal with the error by giving up reading or writing the first buffer head of a command, and retrying the remainder of the I/O. The Linux iSCSI driver attempts to avoid this problem by translating deferred sense data to current sense data for commands sent to disk devices. - Linux kernels 2.2.16 through 2.2.21 and 2.4.0 through 2.4.20 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. - 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. - Linux kernels prior to 2.4.20-8 (Redhat 9 distribution) had a problem of a rare occurrence of data corruption. This data can be buffer cache data as well as raw I/O data. This problem occurs when iSCSI driver sends the I/O request down to TCP. Linux iSCSI driver handles this problem by copying the incoming I/O buffer temporarily in an internal buffer and then sending the copied data down to TCP. This way the iSCSI driver keeps the original data intact. In case, this sent data gets corrupted (this gets detected by turning on CRC), the driver repeats the foregoing process. The iSCSI Driver Version 3.2.1 for Linux is compatible with SN 5400 Series Storage Routers running software version 3.x or greater. It is not compatible with SN 5400 Series Storage Routers running software versions 1.x or 2.x. ---------------------- INSTALLATION PROCEDURE ---------------------- Before you begin, make sure that your Cisco SN 5400 Series Storage Router or other iSCSI target is connected to your network. If another version of the Linux driver is currently installed (it may have been included with your Linux distribution), you will need to remove it before installing a newer driver version. See the installed driver's documentation for detail procedures on removing the driver. 1. Download the linux-iscsi-.tgz file to a working directory (e.g., /usr/src/iscsi) where is the driver version (e.g., 3.2.1). If the directory already exists, remove any files from a previous installation. 2. Change to the working directory created in step 1 and use tar(1) to uncompress the source archive. The archive will contain a subdirectory corresponding to the archive name. For example: cd /usr/src/iscsi tar xvzf linux-iscsi-3.2.1.tgz cd linux-iscsi-3.2.1 Your working directory (/usr/src/iscsi in the example above) now contains the iSCSI package. The package contains the driver and associated files. 3. Compile the iSCSI driver. If your kernel sources are not in the usual place, add 'TOPDIR=/path/to/kernel' or edit the definition of TOPDIR in the makefile. If your kernel configuration file is not in the usual place, add 'KERNEL_CONFIG=/path/to/.config' or edit the definition of KERNEL_CONFIG in the makefile. Compile the driver by entering: make 4. As root, install the driver by entering: make install 5. You may need to verify the order of the boot script because the iSCSI daemon tries to get a TCP connection to each configured SN 5400 Series system and if the network interface has not started by the time the boot script runs, the connection(s) will not occur. Also, if you are using a non-RedHat Linux distribution you may need to edit your boot scripts to properly run the iSCSI setup script. You may also need to verify the order of the shutdown script. 6. Edit /etc/iscsi.conf to setup the correct SN 5400 Series system IP addresses and other required driver parameters. Refer to the Editing /etc/iscsi.conf File section of this document for more information on the settings used in this file. 7. Start the driver by entering the following command. /etc/init.d/iscsi start This should configure devices and cause the iSCSI driver to be loaded. Normal disk commands like mkfs and fsck will work on the iSCSI devices just like they work on a local hard drive. ----------------- REMOVAL PROCEDURE ----------------- 1. While logged in as root, unmount all iSCSI devices and stop the driver by entering: /etc/init.d/iscsi stop 2. De-install the iSCSI driver package by entering: make remove This will delete the appropriate files from /lib/modules and /usr/local/sbin. The configuration files in /etc are not deleted, since they will be needed if another driver version is installed later. 3. Back up one directory level and delete the source code by entering: cd .. rm -rf linux-iscsi- ------------------------ DRIVER UPGRADE PROCEDURE ------------------------ To upgrade the iSCSI driver to the latest version, first deinstall the existing package following the steps given in the Removal Procedure section of the README for the driver you currently have installed. Upon successful deinstallation, install the latest driver as described in the Installation Procedure section of the README for the new driver. ------------------------ KERNEL UPGRADE PROCEDURE ------------------------ Because the iSCSI driver contains a Linux kernel module, the driver must be rebuilt and reinstalled if you make any changes to your Linux kernel. To remove, rebuild, and reinstall the driver, enter the following commands while logged in as root: make remove make clean make make install Because some Linux distributions include versions of this driver in their kernel source tree, recompiling your kernel source tree may create an older version of the iSCSI kernel module. The driver will fail to operate correctly if there is a version mismatch between the iSCSI daemon and the iSCSI kernel module. If your kernel source tree already contains a version of this driver different from the one you have installed, you must disable the iSCSI driver in your kernel configuration before recompiling a new kernel from source. Leaving the iSCSI driver enabled in the kernel configuration during a kernel rebuild may result in a newer iSCSI kernel module being replaced by an older version from the kernel source tree. =============================================================================== CONFIGURING AND USING THE DRIVER =============================================================================== This section describes a number of topics related to configuring and using the iSCSI Driver for Linux. The topics covered include: Starting and Stopping the iSCSI driver Rebooting Linux Device Names Auto-Mounting Filesystems Log Messages Dynamic Driver Reconfiguration Target Portal Failover iSCSI HBA Status Using Multipath I/O Software Making Storage Configuration Changes Target and LUN Discovery Limits Dynamic Target And LUN Discovery Persistent Target Binding Target Authentication Editing The iscsi.conf File iSCSI Commands and Utilities Driver File Listing -------------------------------------- STARTING AND STOPPING THE iSCSI DRIVER -------------------------------------- To manually start the iSCSI driver enter: /etc/init.d/iscsi start 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 ******************************************************************** The directory /proc/scsi/iscsi will contain a file (the controller number) that contains information about the iSCSI devices. To see the iscsi devices currently available on this system, use the utility: /usr/local/sbin/iscsi-ls -l If there are problems loading the iSCSI kernel module, diagnostic information will be placed in /var/log/iscsi.log. To manually stop the iSCSI driver enter: /etc/init.d/iscsi stop When the driver is stopped, the init.d script will attempt to kill all processes using iSCSI devices by first sending them "SIGTERM" and then by sending any remaining processes "SIGKILL". The init.d script will then unmount all iSCSI devices in /etc/fstab.iscsi and kill the iSCSI daemon terminating all connections to iSCSI devices. It is important to note that the init.d script may not be able to successfully unmount filesystems if they are in use by processes that can't be killed. It is recommended that the you manually stop all applications using the filesystem on iSCSI devices before stopping the driver. Filesystems not listed in /etc/fstab.iscsi will not be unmounted by the script and should be manually unmounted prior to a system shutdown. It is very important to unmount all filesystems on iSCSI devices before stopping the iSCSI driver. If the iSCSI driver is stopped while iSCSI devices are mounted, buffered writes may not be committed to disk and file system corruption may occur. --------------- REBOOTING LINUX --------------- The Linux "reboot" command should not be used to reboot the system while iSCSI devices are mounted or being used since the "reboot" command will not execute the iSCSI shutdown script in /etc/rc6.d/ and file system corruption may occur. To safely reboot a Linux system, enter the following command: /sbin/shutdown -r now All iSCSI devices should be unmounted prior to a system shutdown or reboot. See the Starting And Stopping The Driver section of this document for a more detailed description of the unmounting process. ------------ 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 or /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 scans the system to determine the mapping from SCSI device nodes to iSCSI targets, and then creates 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 exist 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. ------------------------- AUTO-MOUNTING FILESYSTEMS ------------------------- Filesystems installed on iSCSI devices cannot be automatically mounted at system reboot due to the fact that the IP network is not yet configured at mount time. However, the driver provides a method to auto-mount these filesystems as soon as the iSCSI devices become available (i.e., after the IP network is configured). To auto-mount a filesystem installed on an iSCSI device, follow these steps: 1. List the iSCSI partitions to be automatically mounted in /etc/fstab.iscsi which has the same format as /etc/fstab. The /etc/fstab.iscsi file will not be overwritten when the driver is installed nor will removing the current version of the driver delete /etc/fstab.iscsi. It is left untouched during an install. 2. For each filesystem on each iscsi device(s), enter the logical volume on which the filesystem resides. The mount points must exist for the filesystems to be mounted. For example, the following /etc/fstab.iscsi entries will mount the two iSCSI devices specified (sda and sdb): ************************************************************************* #device mount FS mount backup fsck #to mount point type options frequency pass /dev/sda /mnt/t0 ext2 defaults 0 0 /dev/sdb /mnt/t1 ext2 defaults 0 0 ************************************************************************* 3. Upon a system restart, the iSCSI startup script invokes the iscsi-mountall script will try to mount iSCSI devices listed in /etc/fstab.iscsi file. iscsi-mountall tries to mount the iSCSI devices for "NUM_RETRIES" (default value 10) number of times, at an interval of "SLEEP_INTERVAL" seconds (default value 1) between each attempt, giving the driver the time to establish a connection with an iSCSI target. The value of these parameters can be changed in the iscsi-mountall script if the devices are not getting configured in the system within the default time periods. Due to variable network delays, targets may not always become available in the same order from one boot to the next. Thus, the order in which iSCSI devices are mounted may vary and may not match the order the devices are listed in /etc/fstab.iscsi You should not assume mounts of iSCSI devices will occur in any particular order. 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. ------------ LOG MESSAGES ------------ The iSCSI driver contains components in the kernel and user level. The log messages from these components are sent to syslog. Based on the syslogd configuration on the Linux host, the messages will be sent to the appropriate destination. For example, if /etc/syslog.conf has the following entry: *.info /var/log/messages then all log messages of level 'info' or higher will be sent to /var/log/messages. If /etc/syslog.conf has the following entry: *.info;kern.none /var/log/messages then all log messages (except kernel messages) of level info or higher will be sent to /var/log/messages. If /etc/syslog.conf has the following entry: kern.* /dev/console then all kernel messages will be sent to the console. All messages from the iSCSI driver when loading the iSCSI kernel module will be placed in /var/log/iscsi.log. The user can also use dmesg(8) to view the log messages. ------------------------------ DYNAMIC DRIVER RECONFIGURATION ------------------------------ Configuration changes can be made to the iSCSI driver without having to stop it or reboot the host system. To dynamically change the configuration of the driver, follow the steps below: 1. Edit /etc/iscsi.conf with the desired configuration changes. 2. Enter the following command: /etc/init.d/iscsi reload This will cause the iSCSI daemon to re-read /etc/iscsi.conf file and to create any new DiscoveryAddress connections it finds. Those discovery sessions will then discover targets and create new target connections. Note that any configuration changes will not affect existing target sessions. For example, removal of a DiscoveryAddress entry from /etc/iscsi.conf will not cause the removal of sessions to targets discovered through this DiscoveryAddress, but it will cause the removal of the discovery session corresponding to the deleted DiscoveryAddress. ---------------------- TARGET PORTAL FAILOVER ---------------------- Some SN 5400 Series Storage Routers have multiple Gigabit Ethernet ports. Those systems may be configured to allow iSCSI target access via multiple paths. When the iSCSI driver discovers targets through a multi-port SN 5400 Series system, it also discovers all the IP addresses that can be used to reach each of those targets. When an existing target connection fails, the iSCSI driver will attempt to connect to that target using the next available IP address. You can also choose a preferred portal to which the iSCSI driver should attempt to connect to when the iSCSI driver is started or whenever automatic portal failover occurs. This is significant in a situation when you want the connection to the targets to be made through a faster network portal (for example, when the I/Os are going through a Gigabit Ethernet interface and you do not prefer the connection to failover to a slower network interface). The preference for portal failover can be specified through the "PreferredPortal" or "PreferredSubnet" parameter in /etc/iscsi.conf. If this preference is set, then on any subsequent failover the driver will first try to failover to the preferred portal or preferred subnet whichever is specified in the conf file. If both preferred portal and preferred subnet entries are present in the conf file then the preferred portal takes precedence. If the preferred portal or preferred subnet is unreachable, then the driver will continuously rotate through the list of available portals until it finds one that is active. The Portal Failover feature is turned on by default and the whole process of failover occurs automatically. You can chose to turn off portal failover by disabling the portal failover parameter in /etc/iscsi.conf. If a target advertises more than one network portal, you can manually switch portals by writing to the HBA's special file in /proc/scsi/iscsi/. For example, if a target advertises two network portals: and If the device is configured with targetId as 0, busId as 0, HBA's host number is 3 and you want to switch the target from to, use the following command: echo "target 0 0 address" > /proc/scsi/iscsi/3 Where the syntax is: echo "target address " > /proc/scsi/iscsi/ The host system must have multiple network interfaces to effectively utilize this failover feature. ---------------- iSCSI HBA STATUS ---------------- 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 iSCSI bus number, target id number, and logical unit number, as well as the IP address, TCP port, and iSCSI TargetName. If an iSCSI session exists, but no LUNs have yet been found for a target, the LUN number field will contain a question mark. If a TCP connection is not currently established, the IP address and port number will both appear as question marks. ---------------------------- USING MULTIPATH I/O SOFTWARE ---------------------------- If a third-party multipath I/O software application is being used in conjunction with the iSCSI driver (e.g., HP Secure Path), it may be necessary to modify the configuration of the driver to allow the multi-pathing software to operate more efficiently. If you are using a multipath I/O application, you may need to set the "ConnFailTimeout" parameter of the iSCSI driver to a smaller value so that SCSI commands will fail more quickly when an iSCSI network connection drops allowing the multipath application to try a different path to for access to the storage device. Also, you may need to set the "MaxDiskCommandTimeout" to a smaller value (e.g., 5 or 10 seconds), so that SCSI commands to unreachable or unresponsive devices will fail more quickly and the multipath software will know to try a different path to the storage device. Multipath support in the iSCSI driver can be turned on by setting Multipath=<"yes" or "portal" or "portalgroup"> in /etc/iscsi.conf. If Multipath=<"yes" or "portal">, then the discovered targets that are configured to allow access via multiple paths will have a separate iSCSI session created for each path (i.e., iSCSI portal). The target portal failover feature should not be used if Multipath=<"yes" or "portal"> since multiple sessions will be established with all available paths. ------------------------------------ MAKING STORAGE CONFIGURATION CHANGES ------------------------------------ Making changes to your storage configuration, including adding or removing targets or LUNs, remapping targets, or modifying target access, may change how the devices are presented to the host operating system. This may require corresponding changes in the iSCSI driver configuration and /etc/vfstab file. It is important to understand the ramifications of SCSI routing service configuration changes on the hosts accessing the associated storage devices. For example, changing the instance configuration may change the device presentation to the host's iSCSI driver, effectively changing the name or number assigned to the device by the host operating system. Certain configuration changes, such as adding or deleting targets, adding or deleting LUNs within a particular target, or adding or deleting entire instances may change the order of the devices presented to the host. Even if the host is only associated with one SCSI routing service instance, the device order could make a difference. Typically, the host operating system assigns drive identifications in the order they are received based on certain criteria. Changing the order of the storage device discovery may result in a changed drive identification. Applications running on the host may require modifications to appropriately access the current drives. If an entire SCSI routing service instance is removed, or there are no targets available for the host, the host's iSCSI driver configuration file must be updated to remove the appropriate reference before restarting the iSCSI driver. If a host's iSCSI configuration file contains an IP address of a SCSI routing service instance that does not exist, or has no targets available for the host, the iSCSI driver will not complete a login and will keep on trying to discover targets associated with this SCSI routing service instance. In general, the following steps are normally required when reconfiguring iSCSI storage: 1. Unmount any filesystems and stop any applications using iSCSI devices. 2. Stop the iSCSI driver by entering: /etc/init.d/iscsi stop 3. Make the appropriate changes to the iSCSI driver configuration file. Remove any references to iSCSI DiscoveryAddresses that have been removed, or that no longer have valid targets for this host. 4. Modify /etc/fstab.iscsi and application configurations as appropriate. 5. Restart the iSCSI driver by entering: /etc/init.d/iscsi start Failure to appropriately update the iSCSI configuration using the above procedure may result in a situation that prevents the host from accessing iSCSI storage resources. ------------------------------- TARGET AND LUN DISCOVERY LIMITS ------------------------------- The bus ID and target ID are assigned by the iSCSI initiator driver whereas the lun ID is assigned by the iSCSI target. The driver provides access to a maximum of 256 bus IDs with each bus supporting 256 targets and each target capable of supporting 256 LUNs. Any discovered iSCSI device will be allocated the next available target ID on bus 0. If a target ID > 256 on bus 0, then a next available target ID on bus 1 will be allocated. If a bus ID > 256 and LUN ID > 256 it will be ignored by the driver and will not be configured in the system. -------------------------------- DYNAMIC TARGET AND LUN DISCOVERY -------------------------------- 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 activate LUNs on all targets. If a new LUN is dynamically added to an existing target on a SCSI routing instance with which the driver has established a connection, then the driver does not automatically activate the new LUN. The user can manually activate the new LUN by executing the following command: echo "scsi add-single-device " > /proc/scsi/scsi where; HBA#: is the controller number present under /proc/scsi/iscsi/ bus-id: is the bus number present on controller . target-id: is the target ID present on ,. LUN: new LUN added dynamically to the target. ------------------------- PERSISTENT TARGET BINDING ------------------------- This feature ensures that the same iSCSI bus and target id number are used for every iSCSI session to a particular iSCSI TargetName, and a Linux SCSI target always maps to the same physical storage device from one reboot to the next. 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 cannot 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. The file /etc/iscsi.bindings is used by the iSCSI daemon to store bindings of iSCSI target names to SCSI target ID's. If the file doesn't exist, it will be created when the driver is started. If an entry exists for a discovered target, the Linux target ID from the entry is assigned to the target. If no entry exists for a discovered target, an entry is written to the file. Each line of the file contains the following fields: BusId TargetId TargetName An example file would be: ***************************************************************************** 0 0 0 1 0 2 ***************************************************************************** Note that the /etc/iscsi.bindings file will permanently contain entries for all iSCSI targets ever logged into from this host. If a target is no longer available to a host you can manually edit the file and remove entries so the obsolete target no longer consumes a SCSI target ID. If you know the iSCSI target name of a target in advance, and you want it to be assigned a particular SCSI target ID, you can add an entry manually. You should stop the iSCSI driver before editing the /etc/iscsi.bindings file. Be careful to keep an entire entry on a single line, with only whitespace characters between the three fields. Do not use a target ID number that already exists in the file. ***************************************************************************** NOTE: iSCSI driver versions prior to 3.2 used the file /var/iscsi/bindings instead of /etc/iscsi.bindings. The first time you start the new driver version, it will change the location and the name of the bindings file to /etc/iscsi.bindings ***************************************************************************** --------------------- TARGET AUTHENTICATION --------------------- The CHAP authentication mechanism provides for two way authentication between the target and the initiator. The authentication feature on the SN 5400 system has to be enabled to make use of this feature. The username and password for both initiator side and target side authentication needs to be listed in /etc/iscsi.conf. The username and password can be specified as global values or can be made specific to the target address. Please refer to the Editing The iscsi.conf File section of this document for a more detailed description of these parameters. --------------------------- EDITING THE ISCSI.CONF FILE --------------------------- The /etc/iscsi.conf file is used to control the operation of the iSCSI driver by allowing the user to configure the values for a number of programmable parameters. These parameters can be setup to apply to specific configuration types or they can be setup to apply globally. The configuration types that are supported are: - DiscoveryAddress = SCSI routing instance IP address with format a.d.c.d or a.b.c.d:n or hostname. - TargetName = Target name in 'iqn' or 'eui' format eg: TargetName = - TargetIPAddress = Target name with format a.b.c.d/n - Subnet = Network portal IP address with format a.b.c.d/n or a.b.c.d&hex - Address = Network portal IP address with format a.b.c.d/32 The complete list of parameters that can be applied either globally or to the configuration types listed above are shown below. Not all parameters are applicable to all configuration types. - Username = CHAP username used for initiator authentication by the target. - OutgoingUsername = < same as above > - Password = CHAP password used for initiator authentication by the target. - OutgoingPassword = < same as above > - IncomingUsername = CHAP username for target authentication by the initiato r. - IncomingPassword = CHAP password for target authentication by the initiato r. - HeaderDigest = Type of header digest support the initiator is requesting of the target. - DataDigest = Type of data digest support the initiator is requesting of the target. - PortalFailover = Enabling/disabling of target portal failover feature. - PreferredSubnet = IP address of the subnet that should be used for a portal failover. - PreferredPortal = IP address of the portal that should be used for a portal failover. - Multipath = Enabling/disabling of multipathing feature. - LoginTimeout = Time interval to wait for a response to a login request to be received from a target before failing a connection attempt. - AuthTimeout = Time interval to wait for a response to a login request containing authentication information to be received from a target before failing a connection attempt. - IdleTimeout = Time interval to wait on a connection with no traffic before sending out a ping. - PingTimeout = Time interval to wait for a ping response after a ping is sent before failing a connection. - ConnFailTimeout = Time interval to wait before failing SCSI commands back to an application for unsuccessful commands. - AbortTimeout = Time interval to wait for a abort command to complete before declaring the abort command failed. - ResetTiemout = Time interval to wait for a reset command to complete before declaring the reset command failed. - InitialR2T = Enabling/disabling of R2T flow control with the target. - MaxRecvDataSegmentLength = Maximum number of bytes that the initiator can receive in an iSCSI PDU. - FirstBurstLength = Maximum number of bytes of unsolicited data the initiator is allowed to send. - MaxBurstLength = Maximum number of bytes for the SCSI payload negotiated by initiator. - TCPWindowSize = Maximum number of bytes that can be sent over a TCP connection by the initiator before receiving an acknowledgement from the target. - Continuous = Enabling/disabling the discovery session to be kept alive. A detailed description for each of these parameters is included in both the man page and the included sample iscsi.conf file. Please consult these sources for examples and more detailed programming instructions. ---------------------------- iSCSI COMMANDS AND UTILITIES ---------------------------- This section gives a description of all the commands and utilities available with the iSCSI driver. - "iscsi-ls" lists information about the iSCSI devices available to the driver. Please refer to the man page for more information. ------------------- DRIVER FILE LISTING ------------------- The following files are present on the system after the successful installation of iSCSI driver: /etc/iscsi.conf /etc/initiatorname.iscsi /etc/init.d/iscsi /etc/rc2.d/S83iscsi /etc/rc3.d/S83iscsi /etc/rc4.d/S83iscsi /etc/rc5.d/S83iscsi /etc/rc0.d/K20iscsi /etc/rc1.d/K20iscsi /etc/rc6.d/K20iscsi /usr/local/sbin/iscsi-mountall /usr/local/sbin/iscsid /usr/local/sbin/iscsi-device /usr/local/sbin/iscsi-iname /usr/local/sbin/iscsi-ls /usr/local/sbin/iscsi-umountall /usr/local/man/man5/iscsi.conf.5 /usr/local/man/man5/iscsi.bindings.5 /usr/local/man/man5/iscsi-ls.5 /usr/local/man/man8/iscsid.8 /etc/iscsi.bindings /lib/modules//kernel/misc/iscsi_mod.o

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