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Re: sync rep & fsync=off

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Grzegorz Jaskiewicz <gj(at)pointblue(dot)com(dot)pl>
Cc: pgsql-hackers(at)postgresql(dot)org, Simon Riggs <simon(at)2ndquadrant(dot)com>
Subject: Re: sync rep & fsync=off
Date: 2011-03-19 20:31:19
Message-ID: AANLkTik55aujGMeujWew2D_3sa+qQxR=h30_iS_5Bw=j@mail.gmail.com (view raw or flat)
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On Sat, Mar 19, 2011 at 11:34 AM, Grzegorz Jaskiewicz
<gj(at)pointblue(dot)com(dot)pl> wrote:
>
> 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.

Well, it's possible.  But I think it'd be worth a look at the code to
see if there's some bad interaction there between the no-fsync code
and the sync-rep code - like, if we don't actually fsync, does the
flush pointer ever get updated?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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