Date: Tue, 15 Aug 2006 13:25:21 -0400
From: Michael Stone <mstone+postgres@mathom.us>
Subject: Re: Postgresql Performance on an HP DL385 and
In-reply-to: <20060815162524.GR27928@pervasive.com>
To: "Jim C. Nasby" <jnasby@pervasive.com>
Cc: pgsql-performance@postgresql.org
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On Tue, Aug 15, 2006 at 11:25:24AM -0500, Jim C. Nasby wrote:
>Well, if the controller is caching with a BBU, I'm not sure that order
>matters anymore, because the controller should be able to re-order at
>will. Theoretically. :) But this is why having some actual data posted
>somewhere would be great.

You're missing the point. It's not a question of what happens once it 
gets to the disk/controller, it's a question of whether the xlog write 
has to compete with some other write activity before the write gets to 
the disk (e.g., at the filesystem level). If you've got a bunch of stuff 
in a write buffer on the OS level and you try to push the xlog write 
out, you may have to wait for the other stuff to get to the controller 
write cache before the xlog does. It doesn't matter if you don't have to 
wait for the write to get from the controller cache to the disk if you 
already had to wait to get to the controller cache. The effect is a 
*lot* smaller than not having a non-volatile cache, but it is an 
improvement. (Also, the difference between ext2 and xfs for the xlog is 
pretty big itself, and a good reason all by itself to put xlog on a 
seperate partition that's small enough to not need journalling.)

Mike Stone