Re: Should pg_stat_bgwriter.buffers_backend_fsync be removed?

On Sat, Apr 26, 2014 at 9:16 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Peter Geoghegan <pg(at)heroku(dot)com> writes:
>> 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.
>
> What's your evidence for that claim?

I don't have any evidence, but I think it unlikely that this is an
occurrence that is seen in the real world. I was not able to see any
instances of it on the entire Heroku fleet at one point a few months
back, for one thing. For another, I have never observed this with any
benchmark, even though pgbench-tools presents buffers_backend_fsync
for each test run. The queue compaction patch completely fixed Greg
Smith's original test case. Even then, I believe it was considered
more of a patch addressing an edge case than anything else. Even the
comments above ForwardFsyncRequest() consider the occurance of backend
fsyncs to be only "theoretically possible".

If anyone is aware of any cases where this is still actually known to
happen in production, I'd like to hear about them. However, ISTM that
if this actually does still happen, those cases would be better served
by surfacing the problem in the logs.

--
Peter Geoghegan