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. Putting Hannes and Ben on CC because I'd like to get their opinion, too. Regards Martin -- Dr. Martin Wilck <mwi...@suse.com>, 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@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel
