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Re: Random performance hit, unknown cause.

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Brian Fehrle" <brianf(at)consistentstate(dot)com>, "Claudio Freire" <klaussfreire(at)gmail(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Random performance hit, unknown cause.
Date: 2012-04-12 19:52:11
Message-ID: 4F86EC1B0200002500046E8E@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-performance
Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
> On Thu, Apr 12, 2012 at 3:41 PM, Brian Fehrle
> <brianf(at)consistentstate(dot)com> wrote:
>> This morning, during our nightly backup process (where we grab a
>> copy of the data directory), we started having this same issue.
>> The main thing that I see in all of these is a high disk wait on
>> the system. When we are performing 'well', the %wa from top is
>> usually around 30%, and our load is around 12 - 15. This morning
>> we saw a load  21 - 23, and an %wa jumping between 60% and 75%.
>>
>> The top process pretty much at all times is the WAL Sender
>> Process, is this normal?
> 
> Sounds like vacuum to me.
 
More particularly, it seems consistent with autovacuum finding a
large number of tuples which had reached their freeze threshold. 
Rewriting the tuple in place with a frozen xmin is a WAL-logged
operation.
 
-Kevin

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