[dm-devel] Configuring Path Groups

[Ray Van Dolson]

On Fri, Apr 06, 2012 at 11:29:47PM -0700, Ray Van Dolson wrote:
> I am setting up multipathing on RHEL6 via iSCSI to an IBM XIV.  My
> iSCSI target looks like this:
> 
> 172.16.10.1:3260,1793 iqn.2005-10.com.xivstorage:003974
> 172.16.8.22:3260,2048 iqn.2005-10.com.xivstorage:003974
> 172.16.10.22:3260,2049 iqn.2005-10.com.xivstorage:003974
> 172.16.8.23:3260,2304 iqn.2005-10.com.xivstorage:003974
> 172.16.10.23:3260,2305 iqn.2005-10.com.xivstorage:003974
> 172.16.8.1:3260,1792 iqn.2005-10.com.xivstorage:003974
> 
> (This is one LUN).  Networks are 172.16.10.0/23 and 172.16.8.0/23, 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.
> 
> Ray

It seems like what I'm after is "prio_callout".  However, it doesn't
seem to be documented in the man pages (though it is in one of the
sample configurations).

I tried setting path_grouping_policy to group_by_prio and then defined
prio_callout to point to a bash script on my system that echo's "2".
This doesn't seem to actually get called though.

Actually, after doing a bit more reading it sounds like prio_callout
was replaced by "prio" in RHEL6's version of the tool.  "prio" doesn't
appear to have an option to call a custom script. :(

Ray