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Message-ID: <4C1AE820.6020506@2ndquadrant.com>
Date: Thu, 17 Jun 2010 23:29:36 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
CC: pgsql-performance@postgresql.org
Subject: Re: wal_synch_method = open_sync safe on RHEL 5.5?
References: <4C1ACE5F.1080802@catalyst.net.nz>
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Mark Kirkwood wrote:
> Now I recall some discussion about this enabling direct io and the 
> general flakiness of this on Linux, so is the option regarded as safe?

No one has ever refuted the claims in 
http://archives.postgresql.org/pgsql-hackers/2007-10/msg01310.php that 
it can be unsafe under a heavy enough level of mixed load on RHEL5.  
Given the performance benefits are marginal on ext3, I haven't ever 
considered it worth the risk.  (I've seen much larger gains on 
Linux+Veritas VxFS).  From what I've seen, recent Linux kernel work has 
reinforced that the old O_SYNC implementation was full of bugs now that 
more work is being done to improve that area.  My suspicion (based on no 
particular data, just what I've seen it tested with) is that it only 
really worked before in the very specific way that Oracle does O_SYNC 
writes, which is different from what PostgreSQL does.

P.S. Be wary of expecting pgbench to give you useful numbers on a single 
run.  For the default write-heavy test, I recommend three runs of 10 
minutes each (-T 600 on recent PostgreSQL versions) before I trust any 
results it gives.  You can get useful data from the select-only test in 
only a few seconds, but not the one that writes a bunch.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us


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Mark Kirkwood wrote:
<blockquote cite="mid:4C1ACE5F.1080802@catalyst.net.nz" type="cite">
  <meta http-equiv="content-type" content="text/html; ">
  <font size="-1"><font face="Helvetica">Now I recall some discussion
about this enabling direct
io and the general flakiness of this on Linux, so is the option
regarded as safe?<br>
  </font></font></blockquote>
<br>
No one has ever refuted the claims in
<a class="moz-txt-link-freetext" href="http://archives.postgresql.org/pgsql-hackers/2007-10/msg01310.php">http://archives.postgresql.org/pgsql-hackers/2007-10/msg01310.php</a> that
it can be unsafe under a heavy enough level of mixed load on RHEL5.&nbsp;
Given the performance benefits are marginal on ext3, I haven't ever
considered it worth the risk.&nbsp; (I've seen much larger gains on
Linux+Veritas VxFS).&nbsp; From what I've seen, recent Linux kernel work has
reinforced that the old O_SYNC implementation was full of bugs now that
more work is being done to improve that area.&nbsp; My suspicion (based on
no particular data, just what I've seen it tested with) is that it only
really worked before in the very specific way that Oracle does O_SYNC
writes, which is different from what PostgreSQL does.<br>
<br>
P.S. Be wary of expecting pgbench to give you useful numbers on a
single run.&nbsp; For the default write-heavy test, I recommend three runs
of 10 minutes each (-T 600 on recent PostgreSQL versions) before I
trust any results it gives.&nbsp; You can get useful data from the
select-only test in only a few seconds, but not the one that writes a
bunch.<br>
<br>
<pre class="moz-signature" cols="72">-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
<a class="moz-txt-link-abbreviated" href="mailto:greg@2ndQuadrant.com">greg@2ndQuadrant.com</a>   <a class="moz-txt-link-abbreviated" href="http://www.2ndQuadrant.us">www.2ndQuadrant.us</a>
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