[dm-devel] rdac priority checker changing priorities

[Lucas Brasilino]

Well, maybe it's a newbie opinion, but looks like that's easier if
each path has a
'fixed value' priority, isn't it ? I've made a little code here that returns
a choosen fixed value to paths and everything goes fine.....

regards
Lucas Brasilino

2009/4/30 Chandra Seetharaman <sekharan at us.ibm.com>:
> Hannes,
>
> I think we need to revisit the priority value we provide for preferred
> path(4) relative to active path (2) and non-preferred(1).
>
> Consider the following scenario:
>
> Access to a lun thru 2 preferred and 2 non-preferred path. Lets call
> path group with preferred paths as pg1 and with non-preferred paths as
> pg2.
>
> Initially pg1 has priority of 8 and pg2 has priority of 2. pg1 is chosen
> and I/O goes thru pg1, all good.
>
> Both the paths in pg1 fails, pg2 has been made the active path group and
> I/O is sent thru that path and since it became "active", its priority
> raises to 6 ( 2 path times (active + non-preferred)).
>
> When one of the paths in pg1 comes back, one would expect the failback
> to happen. It doesn't happen as pg1's priority (4) is smaller than that
> of pg2 (6). Which is not correct.
>
> We can do the same exercise with more than 4 paths also, like 6, 8 etc.,
> and the results are worse.
>
> So, IMO we need to give the disproportionately large number for
> preferred path w.r.t active and non-preferred. What do you think ?
>
> chandra
>
>
>
> On Thu, 2009-04-30 at 08:25 +0200, Hannes Reinecke wrote:
>> Hi Lucas,
>>
>> Lucas Brasilino wrote:
>> > Hi
>> >
>> > I don't know if I'm misundertanding something. I've got an DS4700 and I'm
>> > switching from RDAC[1] to multipath, since it's natively supported in
>> > the distribution I use
>> > here (SLES 10 SP2).
>> >
>> > Since RDAC[1] works perfect, I'm trying to use 'rdac' priority in multipath.
>> >
>> > My /etc/multiconf.conf is quite tiny, since I'm building it step-by-step :-) :
>> >
>> > blacklist {
>> >     devnode "^sda[0-9]*"
>> > }
>> >
>> > defaults {
>> >     user_friendly_names     yes
>> >     prio                    rdac
>> >     path_checker            tur
>> > }
>> >
>> > multipaths {
>> >     multipath {
>> >             wwid    3600a0b8000327b900000107549f85224
>> >             alias   mpath0
>> >     }
>> > }
>> >
>> > I think that using 'prio rdac' makes multipath to use 'mpath_prio_rdac' tool.
>> >
>> >  # multipath -v2 -ll
>> > mpath0 (3600a0b8000327b900000107549f85224) dm-0 IBM,1814      FAStT
>> > [size=140G][features=1 queue_if_no_path][hwhandler=1 rdac]
>> > \_ round-robin 0 [prio=6][active]
>> >  \_ 9:0:0:0  sdb 8:16  [active][ready]
>> > \_ round-robin 0 [prio=1][enabled]
>> >  \_ 10:0:0:0 sdc 8:32  [active][ghost]
>> >
>> > So the first path has priority 6, as I can confirm:
>> >
>> > # mpath_prio_rdac /dev/sdb
>> > 6
>> > # mpath_prio_rdac /dev/sdc
>> > 1
>> >
>> > After the first path (prio=6) failure I get:
>> >
>> > # multipath -v2 -ll
>> > sdb: rdac prio: inquiry command indicates error
>> > mpath0 (3600a0b8000327b900000107549f85224) dm-0 IBM,1814      FAStT
>> > [size=140G][features=1 queue_if_no_path][hwhandler=1 rdac]
>> > \_ round-robin 0 [prio=0][enabled]
>> >  \_ 9:0:0:0  sdb 8:16  [failed][faulty]
>> > \_ round-robin 0 [prio=1][enabled]
>> >  \_ 10:0:0:0 sdc 8:32  [active][ghost]
>> >
>> > Ok.. working great, activating the second path. But after the faulty
>> > path is restored:
>> >
>> > # multipath -v2 -ll
>> > mpath0 (3600a0b8000327b900000107549f85224) dm-0 IBM,1814      FAStT
>> > [size=140G][features=1 queue_if_no_path][hwhandler=1 rdac]
>> > \_ round-robin 0 [prio=2][enabled]
>> >  \_ 9:0:0:0  sdb 8:16  [active][ghost]
>> > \_ round-robin 0 [prio=5][active]
>> >  \_ 10:0:0:0 sdc 8:32  [active][ready]
>> >
>> > Second path is now priority!!! And of course does not fails back! By
>> > the way, my LUN is configured in
>> > DS4700 in sort a way that the first path *is* the path to preferred controller.
>> >
>> > I think path priorities should not change. If so first path goes back
>> > to 'active' status.
>> > Am I misunderstanding something ? Or messing things up?
>> >
>> You are using an old version of multipathing for SLES10 SP2.
>> This had a bug triggering priority inversion on RDAC.
>> Please update to the latest version.
>>
>> Cheers,
>>
>> Hannes
>
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