Configuring Path Groups

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I am setting up multipathing on RHEL6 via iSCSI to an IBM XIV.  My
iSCSI target looks like this:,1793,2048,2049,2304,2305,1792

(This is one LUN).  Networks are and, so
as you can see above there are three target IP's on each subnet.

My RHEL6 host has two physical uplinks... one to each subnet.  When I
configure multipath, by default it creates one big path group with all
of the devices above within it and round-robins.

mpathb (2001738000f860110) dm-2 IBM,2810XIV
size=48G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='queue-length 0' prio=1 status=active
  |- 26:0:0:1 sdb 8:16 active ready running
  |- 25:0:0:1 sdc 8:32 active ready running
  |- 27:0:0:1 sdd 8:48 active ready running
  |- 24:0:0:1 sde 8:64 active ready running
  |- 23:0:0:1 sdf 8:80 active ready running
  `- 22:0:0:1 sdg 8:96 active ready running

This results in less than ideal performance.

Ideally I want to only have one path per subnet active at any given
time, with the other paths only activating if there's a failure.

The only other option I seem to have is to set path_grouping_policy to
failover.  This creates six separate path groups of which only one is
active at a time.

I thought perhaps I could do group_by_prio, but am not sure how I can
manually set priorities for my underlying physical iSCSI devices, and
the default "const" method just gives them all priority 1 so they all
end up in the same path group again.

It looks like perhaps I could configure path groups as I want manually
with dmsetup, but I'm sure there's a better way...

As it is I am currently just configuring my iSCSI initiator to log in
to only one target per subnet.  This gets me the best performance but I
lose some (automated) redundancy.

Thoughts appreciated.


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