On Mon, Nov 20, 2017 at 10:11:02AM +0100, Martin Wilck wrote: > Hi Guan, > > while staring at this code the other day, I realized another possible > issue with your latency prioritizer. > > It will cause significant IO to every path of a map during multipath / > multipathd startup. If any paths really have latencies as long as your > patch considers (up to 100s), or worse if they don't respond at all, > startup may be *massively* delayed or may even never complete. So if we > a storage with two mirrors with a fast and a slow leg (I reckon that's > the scenario this patch was made for), and if we're out of luck and the > slow leg is probed first, we may end up in a situation where the fast > leg, which may be fully up and healthy, is never set up (or with big > delay) because multipathd keeps waiting for the slow leg to respond. > > Similar delays can occur whenever pathinfo(..., DI_PRIO) is called. > Unless I'm overlooking something essential here, that's a really > dangerous thing to do. I believe that before activating this prio > checker for everyone, we need find a way to avoid this scenario. > > By using aio with a reasonable timeout for the latency check rather > then sync IO, we could at least set an upper limit for the time > get_prio takes. That would be a first step. But I don't think that > would be sufficient. > > What we'd really need is an asynchronous priority checker, similar to > the asynchronous path checker. The get_prio() call would return > immediately with some special return code indicating to the caller that > a priority check is running the background. A preliminary prio would be > set for the path in pathinfo(), and multipathd would re-check later (or > get some sort of event) when the priority check has actually been done. > An open question is what multipathd should do wrt path grouping if it > only has preliminary prio values, in particular with group_by_prio. Yeah, you're right. Something like this is necessary. We could have the prioritizers work like the checkers and have them include a context, that the checkers can use to save data. Then could make this work like the directio checker, with its async calls. > Putting Hannes and Ben on CC because I'd like to get their opinion, > too. > > Regards > Martin > > -- > Dr. Martin Wilck <mwilck@xxxxxxxx>, Tel. +49 (0)911 74053 2107 > SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton > HRB 21284 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
