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Re: Spread checkpoint sync

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
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 (view raw or flat)
Thread:
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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