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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: lars <lhofhansl@yahoo.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: Ivan Voras <ivoras@freebsd.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
Date: Tue, 12 Jul 2011 21:16:37 -0400
Message-ID: <21639.1310519797@sss.pgh.pa.us> (raw)
In-Reply-To: <CAMkU=1zr_=QgE7if3cxAv779CwU5xt4iGtn=jMHszxX8dC5ymQ@mail.gmail.com>
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Jeff Janes <jeff.janes@gmail.com> writes:
> On 7/12/11, lars <lhofhansl@yahoo.com> wrote:
>> The fact that a select (maybe a big analytical query we'll run) touching
>> many rows will update the WAL and wait
>> (apparently) for that IO to complete is making a fully cached database
>> far less useful.
>> I just artificially created this scenario.

> I can't think of any reason that that WAL would have to be flushed
> synchronously.

Maybe he's running low on shared_buffers?  We would have to flush WAL
before writing a dirty buffer out, so maybe excessive pressure on
available buffers is part of the issue here.

			regards, tom lane



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