Date: Thu, 18 Jul 2013 12:00:45 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Greg Smith <greg@2ndQuadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>,
 Heikki Linnakangas <hlinnakangas@vmware.com>,
 KONDO Mitsumasa <kondo.mitsumasa@lab.ntt.co.jp>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Improvement of checkpoint IO scheduler for stable
 transaction responses
Message-ID: <20130718160045.GN4165@eldon.alvh.no-ip.org>
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Greg Smith escribió:
> On 7/18/13 11:04 AM, Robert Haas wrote:
> >On a system where fsync is sometimes very very slow, that
> >might result in the checkpoint overrunning its time budget - but SO
> >WHAT?
> 
> Checkpoints provide a boundary on recovery time.  That is their only
> purpose.  You can always do better by postponing them, but you've
> now changed the agreement with the user about how long recovery
> might take.
> 
> And if you don't respect the checkpoint boundary, what you can't do
> is then claim better execution performance than something that did.
> It's always possible to improvement throughput by postponing I/O.
> SO WHAT? If that's OK, you don't need complicated logic to do that.
> Increase checkpoint_timeout.  The system with checkpoint_timeout at
> 6 minutes will always outperform one where it's 5.

I think the idea is to have a system in which most of the time the
recovery time will be that for checkpoint_timeout=5, but in those
(hopefully rare) cases where checkpoints take a bit longer, the recovery
time will be that for checkpoint_timeout=6.

In any case, if the system crashes past minute 5 after the previous
checkpoint (the worst possible timing), the current checkpoint will have
already started, so recovery will take slightly less time because some
flush work had already been done.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services


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