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It is quite common to connect an iSCSI device via two or more network paths for failover reasons. The paths are then joined to one device again with the help of multipath. The iSCSI device must be accessed via the multipath device and must not be accessed via single iSCSI path devices. Otherwise data loss or partition corruption may occur.

Multpath devices can be displayed like so:

root@kvm55:~# multipath -l
350002ac21d622071 dm-4 3PARdata,VV
size=5.0G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=-1 status=active
  |- 6:0:0:0 sdb 8:16 active undef running
  `- 5:0:0:0 sdc 8:32 active undef running
350002ac21d632071 dm-2 3PARdata,VV
size=500G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=-1 status=active
  |- 6:0:0:1 sdd 8:48 active undef running
  `- 5:0:0:1 sde 8:64 active undef running

Some information are especially important and are related to information also displayed by other tools:

The manucaturer ID identifies 3PAR iSCSI storage.

size=5.0G, size=500G
The size may identify which device was intended for what purpose. So far unknown device often can only be differed by their size.

dm-2, dm-4
These DM devices have to be used to access to iSCSI storage. LVM devices will be created using these devices.

6:0:0:0, 5:0:0:0…
The LUN id numbers can be found in log files and in the output of lsscsi. Thus hard disks can be identified.

The multipath device ids help to identify DM devices (see below). The DM device numbers like e.g. 'dm-4' displayed by multipath for this multipath device id have to be identical with DM device numers displayed elswere in order to be sure to access the same multipath device.

Under normal conditions multipath hardly causes trouble. In case the multipath information have to be reread or reassigned, multipath information can be deleted. Afterwards multipath will be restarted:

root@kvm55:~# multipath -F
root@kvm55:~# /etc/init.d/multipath-tools restart

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public/iscsi/multipath.txt · Last modified: 2015/07/12 14:24 by T Kornack