Date: Mon, 3 Mar 2008 11:40:07 -0500 (EST)
From: Greg Smith <gsmith@gregsmith.com>
To: Douglas J Hunley <doug@hunley.homeip.net>
cc: pgsql-performance@postgresql.org
Subject: Re: which is more important? freq of checkpoints or the
	duration of them?
In-Reply-To: <200803030925.02931.doug@hunley.homeip.net>
Message-ID: <Pine.GSO.4.64.0803031126470.10152@westnet.com>
References: <200803030925.02931.doug@hunley.homeip.net>
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On Mon, 3 Mar 2008, Douglas J Hunley wrote:

> In other words, is it ok to checkpoint say, every 5 minutes, if it only 
> last a second or three or better to have checkpoints every 10 minutes 
> that last half a minute?

When checkpoints do too much work at once they will block clients for a 
significant period of time near the end--anywhere from 2 to 8 seconds 
isn't unusual.  Every client on the system will just hang, then they all 
start responding again in a batch when the checkpoint is finished.

With that as the problematic case, if you can keep the duration of the 
checkpoint processing minimal by having them happen more frequently, then 
that's the better approach.  You can't push that interval too small though 
or your system will be continuously checkpointing.

In cases where checkpoints hurt no matter how often you do them, there it 
makes sense to have them as infrequently as possible so at least you 
minimize the number of times that the disruption happens.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD