Date: Fri, 8 Aug 2008 18:42:05 -0400 (EDT)
From: Greg Smith <gsmith@gregsmith.com>
To: Henrik <henke@mac.se>
cc: pgsql-performance@postgresql.org
Subject: Re: Filesystem setup on new system
In-Reply-To: <D1169895-B311-4780-8CA6-C31314235224@mac.se>
Message-ID: <Pine.GSO.4.64.0808071213030.11658@westnet.com>
References: <D1169895-B311-4780-8CA6-C31314235224@mac.se>
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On Thu, 7 Aug 2008, Henrik wrote:

> My first idea was to have one partition on the RAID 10 using ext3 with 
> data=writeback, noatime as mount options.
>
> But I wonder if I should have 2 partitions on the RAID 10 one for the PGDATA 
> dir using ext3 and one partition for XLOGS using ext2.

Really depends on your write volume.  The write cache on your controller 
will keep having a separate xlog disk from being as important as it is 
without one.  If your write volume is really high though, it may still be 
a bottleneck, and you may discover your app runs better with a dedicated 
ext2 xlog disk instead.

The simple version is:

WAL write volume extremely high->dedicated xlog can be better

WAL volume low->more disks for the database array better even if that 
mixes the WAL on there as well

If you want a true answer for which is better, you have to measure your 
application running on this hardware.

> 6 SAS 15K drives in RAID 10 on one of the SAN controllers for database

With only 6 disks available, in general you won't be able to reach the WAL 
as a bottleneck before being limited by seeks on the remaining 4 database 
disks, so you might as well group all 6 together.  It's possible your 
particular application might prefer it the other way though, if you're 
doing a while lot of small writes for example.  I've seen a separate WAL 
handle low-level benchmarks better, but on more real-world loads it's 
harder to run into that situation.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD