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iSCSI is a network protocol defined to allow scsi commands over TCP/IP stack, allowing to hosts I/O block operations like a device storage attached locally. With iSCSI we’ve to difference to basic concepts:

  • iSCSI initiator: is called the scsi client, and can be connected to the server of two ways:
    - Software initiator: Normally is implemented as a module that will used by the network interface and emulate scsi devices. Is the most implementation.
    - Hardware initiator: It use a dedicated hardware to implement iSCSI. The work load of the iSCSI process is handled by this hardware.
  • iSCSI target: references a storage resource located in a iSCSI server.

The servers (targets) have logical units or LUN, that is a number to identify a logical unit storage in the server and the client (initiator) negotiates the connection of a specified LUN with the target. In a iSCSI network each iSCSI element has a unique and permanent iSCSI name and is assigned and address for access. Normally to name an iSCSI element, is followed the IQN (iSCSI qualified name) format:

  • literal iqn
  • date (yyyy-mm) year and month
  • reversed domain name of the authority (org.alpinelinux, com.example, to.yp.cr)
  • “:” prefixing a storage target name specified by the naming authority.

iSCSI is the most commonly protocol used for the SAN (storage area network) because is cheaper than other protocols for network storage like FCoE (Fibre channel over ethernet). The problem of FCoE is that its traffic doesn’t travels above the IP layer of the protocol stack and its necessary special hardware to support FCoE to routed traffic.

SAN’s are very used to make storage devices accessible to the servers so that the devices appear like locally attached devices to the OS.

In this scenario I’ll configure on CentOS 6 an iSCSI target sharing an array disk device and other server as an iSCSI initiator that will map this device as local storage.

Configuring the iSCSI target

- Install the software package:

# yum -y install scsi-target-utils

- Edit target iSCSI configuration:

# vi /etc/tgt/targets.conf
<target iqn.2012-10.net.cpd:san.target01>
backing-store /dev/md0
initiator-address 172.16.201.201
incominguser iscsiadm iscsiadm123
</target>

- Start the iSCSI target daemon and configure to startup at boot system:

# /etc/init.d/tgtd start
# chkconfig --levels 235 tgtd on

- Add iptables rule to allow iSCSI traffic:

# vi /etc/sysconfig/iptables
-A INPUT -m state --state NEW -m tcp -p tcp --dport 3260 -j ACCEPT
# service iptables restart

- check the iSCSI target configuration:

# tgtadm --mode target --op show
Target 1: iqn.2012-10.net.cpd:san.target01
System information:
Driver: iSCSI
State: ready
I_T nexus information:
LUN information:
LUN: 0
Type: controller
SCSI ID: IET 00010000
SCSI SN: beaf10
Size: 0 MB, Block size: 1
Online: Yes
Removable media: No
Prevent removal: No
Readonly: No
Backing store type: null
Backing store path: None
Backing store flags:
Account information:
iSCSIadm
ACL information:
172.16.201.201

Configuring the iSCSI Initiator

- Install the software package:

# yum -y install iscsi-initiator-utils

- Configure the iqn name for the initiator:

# vi /etc/iscsi/initiatorname.iscsi
InitiatorName=iqn.2012-10.net.cpd:san.initiator01
InitiatorAlias=initiator01

- Edit the iSCSI initiator configuration:

# vi /etc/iscsi/iscsid.conf
# To manually startup the session set to "manual". The default is automatic.
node.startup = automatic
# To enable CHAP authentication
node.session.auth.authmethod = CHAP
# To set a CHAP username and password for initiator
node.session.auth.username = iscsiadm
node.session.auth.password = iscsiadm123

- Start iSCSI initiator daemon:

# /etc/init.d/iscsid start
# chkconfig --levels 235 iscsid on

- Discovering targets in our iSCSI server:

# iscsiadm --mode discovery -t sendtargets --portal 172.16.201.200
172.16.201.200:3260,1 iqn.2012-10.net.cpd:san.target01

- Trying to login with the iSCSI LUN:

# iscsiadm --mode node --targetname iqn.2012-10.net.cpd:san.target01 --portal 172.16.201.200 --login
Logging in to [iface: default, target: iqn.2012-10.net.cpd:san.target01, portal: 172.16.201.200,3260] (multiple)
Login to [iface: default, target: iqn.2012-10.net.cpd:san.target01, portal: 172.16.201.200,3260] successful.

With this command is reponsible of the update of  iSCSI targets database for the files located in /var/lib/iscsi/ :

# cat /var/lib/iscsi/send_targets/172.16.201.200,3260/st_config
discovery.startup = manual
discovery.type = sendtargets
discovery.sendtargets.address = 172.16.201.200
discovery.sendtargets.port = 3260
discovery.sendtargets.auth.authmethod = None
discovery.sendtargets.timeo.login_timeout = 15
discovery.sendtargets.use_discoveryd = No
discovery.sendtargets.discoveryd_poll_inval = 30
discovery.sendtargets.reopen_max = 5
discovery.sendtargets.timeo.auth_timeout = 45
discovery.sendtargets.timeo.active_timeout = 30
discovery.sendtargets.iscsi.MaxRecvDataSegmentLength = 32768

- Checking the status session with the target:

# iscsiadm --mode session --op show
tcp: [2] 172.16.201.200:3260,1 iqn.2012-10.net.cpd:san.target01

Mounting automatically the iSCSI partitions at system boot

- Create a new partition to our iSCSI disk and format it:

# fdisk -l
Disk /dev/sdb: 17.2 GB, 17171480576 bytes
64 heads, 32 sectors/track, 16376 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
Disk /dev/sdb doesn't contain a valid partition table
# fdisk /dev/sdb
# mkfs.ext4 /dev/sdb1

- Checking the UUID for disk:

# blkid /dev/sdb1
/dev/sdb1: UUID="71e86162-011d-49f1-9b4a-9f95a277e6b5" TYPE="ext4"

- Add the next entry in /etc/fstab file:

UUID=71e86162-011d-49f1-9b4a-9f95a277e6b5 /mnt/data ext4 _netdev,rw 0 0

With the mount option _netdev the script netfs is responsible of the mount to this device. Without this option, Linux will try to mount this device before load the network support. We have to check that netfs is enabled to run in the default runlevels:

# chkconfig --list netfs
netfs 0:off 1:off 2:off 3:on 4:on 5:on 6:off

- Mounting the new file system:

# mount -a -O _netdev

Initiator parameters configuration: http://www.open-iSCSI.org/docs/README

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