From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
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To: | Greg Smith <greg(at)2ndquadrant(dot)com> |
Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Spread checkpoint sync |
Date: | 2010-12-01 08:50:14 |
Message-ID: | 4CF60C46.4050601@enterprisedb.com |
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Lists: | pgsql-hackers |
On 01.12.2010 06:25, Greg Smith wrote:
> Jeff Janes wrote:
>> I ask because I don't have a mental model of how the pause can help.
>> Given that this dirty data has been hanging around for many minutes
>> already, what is a 3 second pause going to heal?
>
> The difference is that once an fsync call is made, dirty data is much
> more likely to be forced out. It's the one thing that bypasses all other
> ways the kernel might try to avoid writing the data--both the dirty
> ratio guidelines and the congestion control logic--and forces those
> writes to happen as soon as they can be scheduled. If you graph the
> amount of data shown "Dirty:" by /proc/meminfo over time, once the sync
> calls start happening it's like a descending staircase pattern, dropping
> a little bit as each sync fires.
Do you have any idea how to autotune the delay between fsyncs?
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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