On Wed, 2015-07-15 at 13:52 +0200, Hannes Reinecke wrote: > On 07/15/2015 01:35 PM, James Bottomley wrote: > > On Wed, 2015-07-15 at 13:23 +0200, Hannes Reinecke wrote: > >> If dm-mpath encounters an reservation conflict it should not > >> fail the path (as communication with the target is not affected) > >> but should rather retry on another path. > >> However, in doing so we might be inducing a ping-pong between > >> paths, with no guarantee of any forward progress. > >> And arguably a reservation conflict is an unexpected error, > >> so we should be passing it upwards to allow the application > >> to take appropriate steps. > > > > If I interpret the code correctly, you've changed the behaviour from the > > current try all paths and fail them, ultimately passing the reservation > > conflict up if all paths fail to return reservation conflict > > immediately, keeping all paths running. This assumes that the > > reservation isn't path specific because if we encounter a path specific > > reservation, you've altered the behaviour from route around to fail. > > > That is correct. > As mentioned in the path, the 'correct' solution would be to retry > the offending I/O on another path. > However, the current multipath design doesn't allow us to do that > without failing the path first. > If we were just retrying I/O on another path without failing the > path first (and all paths would return a reservation conflict) we > wouldn't know when we've exhausted all paths. > > > The case I think the original code was for is SAN Volume controllers > > which use path specific SCSI-3 reservations effectively to do traffic > > control and allow favoured paths. Have you verified that nothing we > > encounter in the enterprise uses path specific reservations for > > multipath shaping any more? > > > Ah. That was some input I was looking for. > With that patch I've assumed that persistent reservations are done > primarily from userland / filesystem, where the reservation would > effectively be done on a per-LUN basis. > If it's being used from the storage array internally this is a > different matter. > (Although I'd be very interested how this behaviour would play > together with applications which use persistent reservations > internally; GPFS springs to mind here ...) > > But apparently this specific behaviour wasn't seen that often in the > field; I certainly never got any customer reports about mysteriously > failing paths. Have you already got this patch in SLES, if so, for how long? > Anyway. I'll see if I can come up with something to restore the > original behaviour. Or a way of verifying that nothing in the current enterprise uses path specific reservations ... we can change the current behaviour as long as nothing notices. James -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
