On 2015-09-09 10:46:47 -0400, Robert Haas wrote:
> Well, if you're filling ~1 clog page per second, you're doing ~1 fsync
> per second too.  Or if you are not, then you are thrashing the
> progressively smaller and smaller number of clean slots ever-harder
> until no clean pages remain and you're forced to fsync something -
> probably, a bunch of things all at once.

And you're also triggering a lot of other fsyncs. At the very least
you're pretty constantly flushing the WAL, even with synchronous_commit
= off.

> > Like Andres, I'd want to see a more realistic problem case before
> > expending a lot of work here.
> I think the question here isn't whether the problem case is realistic
> - I am quite sure that a pgbench workload is - but rather how much of
> a problem it actually causes.  I'm very sure that individual pgbench
> backends experience multi-second stallls as a result of this.

The fsync causes that? That seems odd. I mean we release the slru
control lock for writing out pages, don't we?

To me it seems the fsyncs are a red herring here, and the problems are,
uh, much bigger. ISTM that cache replacement locking granularity et al
have a much bigger effect than a fsync every now and then.


Andres Freund

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