Red Hat Knowledgebase

DM-Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, DM-Multipath switches to an alternate path. DM-Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM-Multipath can detect loading on the I/O paths and dynamically re-balance the load.

As the dm-multipath needs at least two HBAs or Fibra switches to create at least two paths, it's possible to simulate a enviroment of multipath. One share device can be exported twice by using some third party iscsitarget software for simulating a multipath. But the problem is how to confirm which two devices are bonded into one path group?

To resolve this problem:

Use the command below, For example:

# scsi_id -g -u -s /block/sdN

If the two devices are actually one physical device, the output "unique SCSI identifier" will be the same. For example:

# scsi_id -g -u -s /block/sdb
149455400000000000000000001000000110400000d000000
# scsi_id -g -u -s /block/sdc
149455400000000000000000001000000110400000d000000

The two devices are in one path group.