[dm-devel] Configuring Path Groups

[Ray Van Dolson]

On Fri, Apr 06, 2012 at 11:39:30PM -0700, Ray Van Dolson wrote:
> 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

Found this[1] post and sounds like perhaps prio_callout does still work
but doesn't play nice with bash scripts for lots of valid reasons.

I did try:

devices {
    device {
        vendor          "dummy"
        product         "dummy"
        prio_callout    "/sbin/ray_prio"
    }

    device {
        path_checker tur
        product "2810XIV"
        vendor "IBM"
        rr_min_io 15
        path_grouping_policy group_by_prio
        prio_callout "/sbin/ray_prio /dev/%n"
        path_selector "round-robin 0"
        no_path_retry queue
        failback 15
    }
}

Per the suggestion in the article, but still no go.  Perhaps if I
rewrote my prio callout in C? :)

[1] http://permalink.gmane.org/gmane.linux.kernel.device-mapper.devel/7904