[dm-devel] [PATCH 0/4] path latency prio fixes

[Benjamin Marzinski]

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 at suse.com>, Tel. +49 (0)911 74053 2107
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
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