On 3/25/09 1:07 AM, "Greg Smith" <gsmith(at)gregsmith(dot)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.