[dm-devel] Configuring Path Groups

[Alexander Murashkin]

> 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

See https://www.redhat.com/archives/dm-devel/2012-March/msg00186.html

It seems that prio_callout feature was removed in recent versions of
multipath. BTW Do you get any error message from multipath? In my case see

multipath.conf line NN, invalid keyword: prio_callout

IMHO prio_callout shall be added back even it does not work well in all
cases (just describe when it does not work). Also there are use cases when /
and /usr are on regular disks (for example, we keep OS on local SCSI disks
and use iSCSI for application data).

Best regards,
	Alexander Murashkin