Linux tgtadm: Setup iSCSI Target ( SAN )

Linux target framework (tgt) aims to simplify various SCSI target driver (iSCSI, Fibre Channel, SRP, etc) creation and maintenance. The key goals are the clean integration into the scsi-mid layer and implementing a great portion of tgt in user space.

The developer of IET is also helping to develop Linux SCSI target framework (stgt) which looks like it might lead to an iSCSI target implementation with an upstream kernel component. iSCSI Target can be useful:

a] To setup stateless server / client (used in diskless setups).
b] Share disks and tape drives with remote client over LAN, Wan or the Internet.
c] Setup SAN – Storage array.
d] To setup loadbalanced webcluser using cluster aware Linux file system etc.

In this tutorial you will learn how to have a fully functional Linux iSCSI SAN using tgt framework.

iSCSI target (server)

Storage resource located on an iSCSI server known as a “target”. An iSCSI target usually represents nothing but hard disk storage. As with initiators, software to provide an iSCSI target is available for most mainstream operating systems.

iSCSI initiator (client)

An initiator functions as an iSCSI client. An initiator typically serves the same purpose to a computer as a SCSI bus adapter would, except that instead of physically cabling SCSI devices (like hard drives and tape changers), an iSCSI initiator sends SCSI commands over an IP network.

Debian / Ubuntu Linux Install tgt

Type the following command to install Linux target framework user-space tools:
$ sudo apt-get install tgt

CentOS / RHEL / Red Hat Linux Install tgt

RHEL 5.2 and older version do not have tgt tools. However, RHEL 5.3 (preview version) comes with tgt tools.

tgtadm – Linux SCSI Target Administration Utility

tgtadm is used to monitor and modify everything about Linux SCSI target software: targets, volumes, etc. This tool allows a system to serve block-level SCSI storage to other systems that have a SCSI initiator. This capability is being initially deployed as a Linux iSCSI target, serving storage over a network to any iSCSI initiator.

Start tgtd

To start the tgtd, enter:
# /usr/sbin/tgtd
Under RHEL 5.3 to start the tgtd service, enter:
# /etc/init.d/tgtd start

Define an iscsi target name

The following example creates a target with id 1 (the iqn is 19 iqn.2001-04.com.example:storage.disk2.amiens.sys1.xyz) and adds a 20 logical unit (backed by /dev/hdc1) with lun 1.
# tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.2001-04.com.example:storage.disk2.amiens.sys1.xyz

To view the current configuration, enter:
# tgtadm --lld iscsi --op show --mode target
Sample output:

Target 1: iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz
    System information:
        Driver: iscsi
        Status: running
    I_T nexus information:
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: deadbeaf1:0
            SCSI SN: beaf10
            Size: 0
            Online: No
            Poweron/Reset: Yes
            Removable media: No
            Backing store: No backing store
    Account information:
    ACL information:

Add a logical unit to the target (/dev/sdb1):
# tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 1 -b /dev/sdb1

A note about home computer / test system

Most production boxes will only use iSCSI root with real iSCSI devices, but for testing purposes it can be quite useful to set up an iSCSI target on your image server. This is useful for testing and learning iSCSI target and iSCSI initiator at home, simply use filesystem for testing purpose. Use dd command to create diskbased filesystem:
# dd if=/dev/zero of=/fs.iscsi.disk bs=1M count=512
Add /fs.iscsi.disk as a logical unit to the target:
# tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 1 -b /fs.iscsi.disk
Now, you should able to view details:
# tgtadm --lld iscsi --op show --mode target
Sample output:

Target 1: iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz
    System information:
        Driver: iscsi
        Status: running
    I_T nexus information:
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: deadbeaf1:0
            SCSI SN: beaf10
            Size: 0
            Online: No
            Poweron/Reset: Yes
            Removable media: No
            Backing store: No backing store
        LUN: 1
            Type: disk
            SCSI ID: deadbeaf1:1
            SCSI SN: beaf11
            Size: 512M
            Online: Yes
            Poweron/Reset: Yes
            Removable media: No
            Backing store: /fs.iscsi.disk
    Account information:
    ACL information:

A Note About Selinux

Renout Gerrits adds – On RHEL and friends you will run into problems if selinux is enabled if using files. While adding a logicalunit to a target you will get the error: ‘tgtadm: invalid request’. A bit of a misguiding error. fixed by setting the correct context:
# semanage fcontext -a -t tgtd_var_lib_t /fs.iscsi.disk
# restorecon -Rv /fs.iscsi.disk

Accept iSCSI Target

To enable the target to accept any initiators, enter:
# tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL
This should open network port # 3260:
# netstat -tulpn | grep 3260
Sample output:

tcp        0      0 0.0.0.0:3260            0.0.0.0:*               LISTEN      27328/tgtd
tcp6       0      0 :::3260                 :::*                    LISTEN      27328/tgtd      

And you are done. Your system is configured as iSCSI Target. Remote client computer can access this computers hard disk over network. Your can use cluster aware filesystem to setup real shared storage for small business. Open TCP port 3260 in your firewall, if required.

How do I access iSCSI Target (server) via iSCSI initiator (client)?

See detailed os specific iSCSI initiator instuctions:

1. RHEL 4 or RHEL 5 Linux iSCSI initiator tutorial.
2. Debian Linux iSCSI initiator tutorial.
3. FreeBSD iSCSI initiator tutorial.
4. Windows iSCSI initiator tutorial.

Following is a quick way to access iSCSI target, under RHEL 5. Let us say your server iSCSI Target IP is 192.168.1.2. Type the following command to discover targets at a given IP address such as 192.168.1.2 (use 127.0.0.1 if you are testing it from same computer):
# iscsiadm --mode discovery --type sendtargets --portal 192.168.1.2
OR
# iscsiadm --mode discovery --type sendtargets --portal 127.0.0.1
Sample output:

127.0.0.1:3260,1 iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz

Login to the iscsi target session:
# iscsiadm --mode node --targetname iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz --portal 192.168.1.2:3260 --login
OR
# iscsiadm --mode node --targetname iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz --portal 127.0.0.1:3260 --login
Verify that login was successful:
# tail -f /var/log/messages
Sample output:

Nov 11 07:34:04 vivek-desktop kernel: [ 9039.562312] scsi 6:0:0:1: Direct-Access     IET      VIRTUAL-DISK     0001 PQ: 0 ANSI: 5
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572268] sd 6:0:0:1: [sdc] 1048576 512-byte hardware sectors (537 MB)
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572374] sd 6:0:0:1: [sdc] Write Protect is off
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572530] sd 6:0:0:1: [sdc] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572715] sd 6:0:0:1: [sdc] 1048576 512-byte hardware sectors (537 MB)
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572790] sd 6:0:0:1: [sdc] Write Protect is off
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572940] sd 6:0:0:1: [sdc] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.572946]  sdc: unknown partition table
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.573492] sd 6:0:0:1: [sdc] Attached SCSI disk
Nov 11 07:34:04 vivek-desktop kernel: [ 9039.573593] sd 6:0:0:1: Attached scsi generic sg4 type 0

sdc is new scsi disk. You may need to restart iSCSI to probe partition and check disks:
# service iscsi restart
# partprobe
# fdisk -l
You can now create parition and mount file system using usual fdisk and mkfs.ext3 commands:
# fdisk /dev/sdc
# mkfs.ext3 /dev/sdc1mkdir /iscsi
# mkdir /iscsi
# mount /dev/sdc1 /iscsi
# df -H
Sample output:

Filesystem             Size   Used  Avail Use% Mounted on
/dev/sda2               99G    30G    64G  32% /
tmpfs                  1.1G      0   1.1G   0% /lib/init/rw
varrun                 1.1G   361k   1.1G   1% /var/run
varlock                1.1G      0   1.1G   0% /var/lock
udev                   1.1G   2.9M   1.1G   1% /dev
tmpfs                  1.1G   312k   1.1G   1% /dev/shm
/dev/sda1              105G    32G    73G  31% /media/sda1
/dev/sda5              294G   275G    20G  94% /share
/dev/sdb2              247G   119G   116G  51% /disk1p2
/dev/sdc1              520M    11M   483M   3% /iscsi

References:

• man page tgtadm
• tgt project
• iSCSI article from the wikipedia