Message-ID: <4F51661A.4090903@fuzzy.cz>
Date: Sat, 03 Mar 2012 01:30:18 +0100
From: Tomas Vondra <tv@fuzzy.cz>
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To: Claudio Freire <klaussfreire@gmail.com>
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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On 2.3.2012 03:05, Claudio Freire wrote:
> On Thu, Mar 1, 2012 at 10:13 PM, Tomas Vondra <tv@fuzzy.cz> wrote:
>>
>> Maybe. I still am not sure how fsync=off affects the eviction in your
>> opinion. I think it does not (or just very remotely) and you were saying
>> the opposite. IMHO the eviction of (dirty) buffers is either very fast
>> or slow, no matter what the fsync setting is.
> 
> I was thinking page cleanup, but if you're confident it doesn't happen
> on a read-only database, I'd have to agree on all your other points.
> 
> I have seen a small amount of writes on a read-only devel DB I work
> with, though. Usually in the order of 100kb/s writes per 10mb/s reads
> - I attributed that to page cleanup. In that case, it can add some
> wait time to fsync, even though it's really a slow volume of writes.
> If you're right, I'm thinking, it may be some other thing... atime
> updates maybe, I'd have to check the filesystem configuration I guess.

I'd guess those writes were caused by hint bits (~ page cleanup, but
that's a one-time thing and should be fixed by VACUUM FREEZE right after
the load). Or maybe it was related to runtime stats (i.e. pgstat).

T.