How To Configure iSCSI initiator with multipathing

For configurations where both paths to the iSCSI target travel over different networks-or-subnets

1. Config the first path through one of you network interfaces:

# service iscsid start
# chkconfig iscsid on
# iscsiadm -m discovery -t st -p -P 1
# iscsiadm -m discovery -t st -p -l

2. After logging into the target you should see new SCSI block devices created, verify this by executing fdisk -l:

# partprobe
# fdisk -l
3. Config the second path through eth1:

# iscsiadm -m discovery -t st -p -P 1
# iscsiadm -m discovery -t st -p -l

For configurations where both paths to the iSCSI target travel over the same network and subnet

• 1. Configure the iSCSI interfaces by creating iSCSI iface bindings for all interfaces and binding by network device name (eth0, alias, vlan name, etc) or MAC address:
  
  
  # service iscsid start
  # chkconfig iscsid on
  # iscsiadm -m iface -I iscsi-eth0 -o new
  # iscsiadm -m iface -I iscsi-eth0 -o update -n iface.net_ifacename -v eth0
  # iscsiadm -m iface -I iscsi-eth1 -o new
  # iscsiadm -m iface -I iscsi-eth1 -o update -n iface.net_ifacename -v eth1
  
  
  2. Next, verify your targets are available and log in:
  
  
  # iscsiadm -m discovery -t st -p -I iscsi-eth0 -I iscsi-eth1 -P 1
  # iscsiadm -m discovery -t st -p -I iscsi-eth0 -I iscsi-eth1 -l
  
  
  3. After logging into the target you should see new SCSI block devices created, verify this by executing fdisk -l:
  
  
  # partprobe
  # fdisk -l
  
  
  
  
  Each LUN has a different World Wide Identifier (WWID.) Each scsi block device with the same WWID is a different path to the same LUN. To verify the WWIDs perform the following:
  
  
  # scsi_id -gus /block/sd
  
  
  Configuring Multipath:
  
  
  After configuring the iSCSI layer Multipath must be configured via /etc/multipath/multipath.com. Please note that different SAN vendors will their own recommendations for configuring the multipath.conf file; their recommendations should be used if they are provided. For more information on the specific settings for your NAS please contact you hardware vendor.
  
  
  1. Make the following changes to /etc/multipath.conf to set up a simple Multipath configuration with default settings:
  
  
  
  
  
  
  * Un-comment the "defaults" stanza by removing the hash symbols on the following lines:
  
  
  defaults {
  user_friendly_names yes
  }
  
  
  * Comment-out the "blacklist" stanza by putting hash symbols on the following lines:
  
  
  # blacklist {
  # devnode "*"
  # }
  
  
  
  
  For more information on device mapper multipath please refer to: Using Device-Mapper Multipath
  
  
  2. Save the changes to multipath.conf. Start multipath and ensure that it is configured to start at boot time:
  
  
  # service multipathd start
  # chkconfig multipathd on
  
  
  3. After starting the multipath daemon the multipath command can be used to view your multipath devices. Example output is as follows:
  
  
  mpath0 (1IET_00010001) dm-4 IET,VIRTUAL-DISK
  [size=10G][features=0][hwhandler=0][rw]
  \_ round-robin 0 [prio=1][active]
  \_ 6:0:0:1 sdf 8:80 [active][ready]
  \_ 7:0:0:1 sdh 8:112 [active][ready]
  
  
  4. Using the mpath psuedo-device for the multipathed storage, create a partition and inform the kernel of the change:
  
  
  # fdisk /dev/mapper/mpath0
  # partprobe
  
  
  5. Use the kpartx command to inform multipath of the new partition:
  
  
  # kpartx -a /dev/mapper/mapth0
  
  
  6. Device mapper will then create a new mpath pseudo device. Example:
  
  
  /dev/mapper/mapth0p1
  
  
  7. Create a file system on the multipathed storage and mount it:
  
  
  # mkfs.ext3 /dev/mapper/mpath0p1
  # mount /dev/mapper/mpath0p1 /
  
  
  8. With the storage mounted begin failover testing. The following is an example of failover testing via a cable-pull on eth1:
  
  
  * Use the mulitpath command to verify that all paths are up. Example output:
  
  
  mpath0 (1IET_00010001) dm-4 IET,VIRTUAL-DISK
  [size=10G][features=0][hwhandler=0][rw]
  \_ round-robin 0 [prio=1][active]
  \_ 6:0:0:1 sdf 8:80 [active][ready]
  \_ 7:0:0:1 sdh 8:112 [active][ready]
  
  
  * Pull the cable on eth1. Verify the path is failed with multipath -ll. Example output:
  
  
  mpath0 (1IET_00010001) dm-4 IET,VIRTUAL-DISK
  [size=10G][features=0][hwhandler=0][rw]
  \_ round-robin 0 [prio=1][active]
  \_ 6:0:0:1 sdf 8:80 [active][ready]
  \_ 7:0:0:1 sdh 8:112 [faulty][failed]
  
  
  
  
  9. The final step in the process is tuning failover timing.
  
  
   With the default timeouts in /etc/iscsi/iscsi.conf multipath failover takes about 1.5 minutes. 
   Some users of multipath and iSCSI want lower timeourts so that I/O doesn't remain queued for long periods of time.