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. Anyway. I'll see if I can come up with something to restore the original behaviour. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
