| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Gregory Stark <stark(at)enterprisedb(dot)com> |
| Cc: | Luke Lonergan <llonergan(at)greenplum(dot)com>, Carlo Stonebanks <stonec(dot)register(at)sympatico(dot)ca>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: random_page_costs - are defaults of 4.0 realistic for SCSI RAID 1 |
| Date: | 2007-09-10 22:03:31 |
| Message-ID: | 1189461811.28581.17.camel@dogma.ljc.laika.com |
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| Lists: | pgsql-performance |
On Mon, 2007-09-10 at 22:44 +0100, Gregory Stark wrote:
> What I don't understand is the bit about "until Postgres gets AIO + the
> ability to post multiple concurrent IOs on index probes". Even with AIO your
> seek times are not going to be improved by wide raid stripes. And you can't
> possibly find the page at level n+1 before you've looked at the page at level
> n. Do you mean to be able to probe multiple index keys simultaneously? How
> does that work out?
>
I think he's referring to mirrors, in which there are multiple spindles
that can return a requested block. That could mitigate random I/O, if
the I/O is asynchronous and something intelligent (OS or controller) can
schedule it.
Regards,
Jeff Davis
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