Re: PG8 Tuning

Alvaro Herrera wrote:
> On Tue, Aug 16, 2005 at 09:12:31AM -0700, Josh Berkus wrote:
>
>
>>However, you are absolutely correct in that it's *relative* advice, not
>>absolute advice. If, for example, you're using a $100,000 EMC SAN as your
>>storage you'll probably be better off giving it everything and letting its
>>controller and cache handle disk allocation etc. On the other hand, if
>>you're dealing with the 5 drives in a single Dell 6650, I've yet to encounter
>>a case where a separate xlog disk did not benefit an OLTP application.
>
>
> I've been asked this a couple of times and I don't know the answer: what
> happens if you give XLog a single drive (unmirrored single spindle), and
> that drive dies? So the question really is, should you be giving two
> disks to XLog?
>

I can propose a simple test. Create a test database. Run postgres,
insert a bunch of stuff. Stop postgres. Delete everything in the pg_xlog
directory. Start postgres again, what does it do?

I suppose to simulate more of a failure mode, you could kill -9 the
postmaster (and all children processes) perhaps during an insert, and
then delete pg_xlog.

But I would like to hear from the postgres folks what they *expect*
would happen if you ever lost pg_xlog.

What about something like keeping pg_xlog on a ramdisk, and then
rsyncing it to a hard-disk every 5 minutes. If you die in the middle,
does it just restore back to the 5-minutes ago point, or does it get
more thoroughly messed up?
For some people, a 5-minute old restore would be okay, as long as you
still have transaction safety, so that you can figure out what needs to
be restored.

John
=:->