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Re: sync rep & fsync=off

From: Grzegorz Jaskiewicz <gj(at)pointblue(dot)com(dot)pl>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, Simon Riggs <simon(at)2ndquadrant(dot)com>
Subject: Re: sync rep & fsync=off
Date: 2011-03-19 15:34:18
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Lists: pgsql-hackers
On 18 Mar 2011, at 21:12, Robert Haas wrote:

> While investigating Simon's complaint about my patch of a few days
> ago, I discovered that synchronous replication appears to slow to a
> crawl if fsync is turned off on the standby.
> I'm not sure why this is happening or what the right behavior is in
> this case, but I think some kind of adjustment is needed because the
> current behavior is quite surprising.
We have few servers here running 8.3. And few weeks ago I had to populate one database with quite a number of entries. 
I have script that does that, but it takes a while. I decided to turn fsck to off. Oddly enough, the server started to crawl quite badly, load was very high. 
That was 8.3 on rhel 5.4. 

My point is, it is sometimes bad combination of disks and controllers that does that. Not necessarily software. fsync off doesn't always mean that things are going to fly, it can cause it to expose hardware bottlenecks much quicker. 

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