Hi Hannes, Please see the attached file for the real example. Can I go ahead and generate a patch to increase the priority of the preferred path to say, 50 ? chandra On Mon, 2009-05-04 at 12:43 +0200, Hannes Reinecke wrote: > Hi Chandra, > > Chandra Seetharaman wrote: > > 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. > > > Is this really a valid case? > This means we'll have a setup like this: > > rdac > pg1 > sda failed > sdb failed > pg2 > sdc active > sdd active > > Correct? > So, given your assumptions, the proposed scenario would be represented > like this: > > rdac > pg1 > sda active > sdb failed > pg2 > sdc active > sdd active > > So it is really a good idea to switch paths in this case? The 'sdb' > path would not be reachable here, so any path switch command wouldn't > have been received, either. I'm not sure _what_ is going to happen > when we switch paths now and sdb comes back later; but most likely > the entire setup will be messed up then: > sda (pref & owned) 6 > sdb 0 > sdc (sec) 1 > sdd (sec & owned) 3 > and we'll be getting the path layout thoroughly jumbled then. > So I don't really like this idea. We should only be switching > paths when _all_ paths of a path group become available again. > Providing not all paths have failed in the active group, of course. > Then we should be switching paths regardless. > > Cheers, > > Hannes
$ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=12][active] \_ 2:0:1:4 sdu 65:64 [active][ready] \_ 2:0:0:4 sdp 8:240 [active][ready] \_ round-robin 0 [prio=2][enabled] \_ 1:0:1:4 sdk 8:160 [active][ghost] \_ 1:0:0:4 sdf 8:80 [active][ghost] $ # disabled one preferred path $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 sdp: rdac prio: inquiry command indicates error 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=6][active] \_ 2:0:1:4 sdu 65:64 [active][ready] \_ 2:0:0:4 sdp 8:240 [failed][faulty] \_ round-robin 0 [prio=2][enabled] \_ 1:0:1:4 sdk 8:160 [active][ghost] \_ 1:0:0:4 sdf 8:80 [active][ghost] $ # ALL GOOD $ # disabled another preferred path $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 sdu: rdac prio: inquiry command indicates error sdp: rdac prio: inquiry command indicates error 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=0][enabled] \_ 2:0:1:4 sdu 65:64 [failed][faulty] \_ 2:0:0:4 sdp 8:240 [failed][faulty] \_ round-robin 0 [prio=6][active] \_ 1:0:1:4 sdk 8:160 [active][ready] \_ 1:0:0:4 sdf 8:80 [active][ready] $ # failed over to the non-preferred path $ # that is good $ # disabled a non-preferred path $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 sdu: rdac prio: inquiry command indicates error sdp: rdac prio: inquiry command indicates error sdk: rdac prio: inquiry command indicates error 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=0][enabled] \_ 2:0:1:4 sdu 65:64 [failed][faulty] \_ 2:0:0:4 sdp 8:240 [failed][faulty] \_ round-robin 0 [prio=3][active] \_ 1:0:1:4 sdk 8:160 [failed][faulty] \_ 1:0:0:4 sdf 8:80 [active][ready] $ # all good $ # enabled a non-preferred path $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 sdu: rdac prio: inquiry command indicates error sdp: rdac prio: inquiry command indicates error 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=0][enabled] \_ 2:0:1:4 sdu 65:64 [failed][faulty] \_ 2:0:0:4 sdp 8:240 [failed][faulty] \_ round-robin 0 [prio=6][active] \_ 1:0:1:4 sdk 8:160 [active][ready] \_ 1:0:0:4 sdf 8:80 [active][ready] $ # Good $ # enabled a preferred path. $ # expected failback to the preferred path group $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 sdp: rdac prio: inquiry command indicates error 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=4][enabled] \_ 2:0:1:4 sdu 65:64 [active][ghost] \_ 2:0:0:4 sdp 8:240 [failed][faulty] \_ round-robin 0 [prio=6][active] \_ 1:0:1:4 sdk 8:160 [active][ready] \_ 1:0:0:4 sdf 8:80 [active][ready] $ # no. failback did not happen. [see the first path group still states "ghost"] $ # the reason is that the priority of the preferred path group is less than $ # that of the non-preferred path group. $ # Basically, non-preferred path is used even though one preferred path is available $ # which is not correct $ # wait for a a minute, may be $ sleep 60 $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 sdp: rdac prio: inquiry command indicates error 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=4][enabled] \_ 2:0:1:4 sdu 65:64 [active][ghost] \_ 2:0:0:4 sdp 8:240 [failed][faulty] \_ round-robin 0 [prio=6][active] \_ 1:0:1:4 sdk 8:160 [active][ready] \_ 1:0:0:4 sdf 8:80 [active][ready] $ # nope... failback didn't happen. $ # enabled the other preferred path. $ # only now the failback happens. $ $ multipath -ll 3600a0b800011a1ee00003f834a3f7a65 3600a0b800011a1ee00003f834a3f7a65 dm-0 IBM,1815 FAStT [size=10G][features=1 queue_if_no_path][hwhandler=1 rdac] \_ round-robin 0 [prio=12][active] \_ 2:0:1:4 sdu 65:64 [active][ready] \_ 2:0:0:4 sdp 8:240 [active][ready] \_ round-robin 0 [prio=2][enabled] \_ 1:0:1:4 sdk 8:160 [active][ghost] \_ 1:0:0:4 sdf 8:80 [active][ghost] $ exit
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