Horton

Re: Raid 10 chunksize

Scott Carey <scott@richrelevance.com>

View thread View on postgr.es
From: Scott Carey <scott@richrelevance.com>
To: Greg Smith <gsmith@gregsmith.com>, Mark Kirkwood <markir@paradise.net.nz>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Date: 2009-03-25T16:16:33Z
Lists: pgsql-performance 
On 3/25/09 1:07 AM, "Greg Smith" <gsmith@gregsmith.com> wrote:

> On Wed, 25 Mar 2009, Mark Kirkwood wrote:
> 
>> I'm thinking that the raid chunksize may well be the issue.
> 
> Why?  I'm not saying you're wrong, I just don't see why that parameter
> jumped out as a likely cause here.
> 

If postgres is random reading or writing at 8k block size, and the raid
array is set with 4k block size, then every 8k random i/o will create TWO
disk seeks since it gets split to two disks.   Effectively, iops will be cut
in half.