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Re: Increasing WAL usage followed by sudden drop

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: delongboy <sdelong(at)saucontech(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Increasing WAL usage followed by sudden drop
Date: 2012-08-17 18:42:27
Message-ID: CAMkU=1xo=o8+wA5Pa=P18OcdBwKx0J=0PCy+HBUDVPsazX3i4Q@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Fri, Aug 17, 2012 at 10:53 AM, delongboy <sdelong(at)saucontech(dot)com> wrote:
>
> Josh Berkus wrote
>>
>>> We are not doing anything to postgres that would cause the rise and
>>> drop.
>>> Data base activity is pretty consistent.  nor are we doing any kind
>>> of
>>> purge.  This week the drop occurred after 6 days.  We are thinking it
>>> must
>>> be some kind of internal postgres activity but we can't track it
>>> down.
>>
>> Well, we certainly can't figure it out on this list by blind guessing ...
>>
>
> Have to agree with you there.  Unfortunately this is where we've ended up.
> What can we look at and/or show that would help us to narrow it down?  Is
> there anyway we can look into the wal file and see exactly what is being
> sent?  I think this might actually give us the most insight.

Maybe there is an easier way, but one thing would be to compile a test
server (of the same version as the production) with WAL_DEBUG defined
in src/include/pg_config_manual.h, turn on the wal_debug guc, and
crank up trace_recovery_messages.  Then replay the WAL log files from
production through this test server and see what it logs.  That
requires that you have

Easier would to be turn on wal_debug and watch the server log as the
WAL logs are generated, instead of recovered, but you probably don't
want to do that on production.  So you would need a workload generator
that also exhibits the phenomenon of interest.

Cheers,

Jeff


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