Re: Should pg_stat_bgwriter.buffers_backend_fsync be removed?

On 4/26/14, 9:42 PM, Peter Geoghegan wrote:
> Backend fsyncs are theoretically still possible after the fsync
> request queue compaction patch (which was subsequently back-patched to
> all supported release branches). However, I'm reasonably confident
> that that patch was so effective as to make a backend fsync all but
> impossible. As such, it seems like the buffers_backend_fsync column in
> the pg_stat_bgwriter view is more or less obsolete.
>
> I suggest removing it for 9.5, and instead logging individual
> occurrences of backend fsync requests within ForwardFsyncRequest(). It
> seems fair to treat that as an anomaly to draw particular attention
> to.

All else equal, I don't like the idea of removing this from pg_stat_bgwriter. Being able to look there and see if this is occurring since last stats reset is much easier than grepping logfiles.

I don't have an issue with logging it, though I think we need to be careful to ensure we don't go crazy if something happens in the system where suddenly all backends are fsyncing. If that happens you're going to have major IO problems and trying to log thousands (or more) of extra entries is just going to make it worse.
--
Jim C. Nasby, Data Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net