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Message-ID: <4C6A4B71.5040909@2ndquadrant.com>
Date: Tue, 17 Aug 2010 04:42:25 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Bruce Momjian <bruce@momjian.us>
CC: Scott Carey <scott@richrelevance.com>, Kenneth Cox <kenstir@gmail.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Advice configuring ServeRAID 8k for performance
References: <201008170131.o7H1VPc26427@momjian.us>
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Bruce Momjian wrote:
> Scott Carey wrote:
>   
>> Don't ever have WAL and data on the same OS volume as ext3.
>>
>> ...
>> One partition for WAL, one for data.  If using ext3 this is essentially
>> a performance requirement no matter how your array is set up underneath.
>>     
>
> Do we need to document this?
>   

Not for 9.0.  What Scott is suggesting is often the case, but not 
always; I can produce a counter example at will now that I know exactly 
which closets have the skeletons in them.  The underlying situation is 
more complicated due to some limitations to the whole "spread 
checkpoint" code that is turning really sour on newer hardware with 
large amounts of RAM.  I have about 5 pages of written notes on this 
specific issue so far, and that keeps growing every week.  That's all 
leading toward a proposed 9.1 change to the specific fsync behavior.  
And I expect to dump a large stack of documentation to support that 
patch that will address this whole area.  I'll put the whole thing onto 
the wiki as soon as my 9.0 related work settles down.

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


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Bruce Momjian wrote:
<blockquote cite="mid:201008170131.o7H1VPc26427@momjian.us" type="cite">
  <pre wrap="">Scott Carey wrote:
  </pre>
  <blockquote type="cite">
    <pre wrap="">Don't ever have WAL and data on the same OS volume as ext3.

...
One partition for WAL, one for data.  If using ext3 this is essentially
a performance requirement no matter how your array is set up underneath.
    </pre>
  </blockquote>
  <pre wrap=""><!---->
Do we need to document this?
  </pre>
</blockquote>
<br>
Not for 9.0.&nbsp; What Scott is suggesting is often the case, but not
always; I can produce a counter example at will now that I know exactly
which closets have the skeletons in them.&nbsp; The underlying situation is
more complicated due to some limitations to the whole "spread
checkpoint" code that is turning really sour on newer hardware with
large amounts of RAM.&nbsp; I have about 5 pages of written notes on this
specific issue so far, and that keeps growing every week.&nbsp; That's all
leading toward a proposed 9.1 change to the specific fsync behavior.&nbsp;
And I expect to dump a large stack of documentation to support that
patch that will address this whole area.&nbsp; I'll put the whole thing onto
the wiki as soon as my 9.0 related work settles down.<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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