Re: pg_xlog on data partition with BBU RAID

On Fri, 2006-06-09 at 14:41, Jim C. Nasby wrote:
> AFAIK, the reason why seperating pg_xlog from the base files provides so
> much performance is because the latency on pg_xlog is critical: a
> transaction can't commit until all of it's log data is written to disk
> via fsync, and if you're trying to fsync frequently on the same drive as
> the data tables are on, you'll have a big problem with the activity on
> the data drives competing with trying to fsync pg_xlog rapidly.
>
> But if you have a raid array with a battery-backed controller, this
> shouldn't be anywhere near as big an issue. The fsync on the log will
> return very quickly thanks to the cache, and the controller is then free
> to batch up writes to pg_xlog. Or at least that's the theory.
>
> Has anyone actually done any testing on this? Specifically, I'm
> wondering if the benefit of adding 2 more drives to a RAID10 outweighs
> whatever penalties there are to having pg_xlog on that RAID10 with all
> the rest of the data.

I tested it WAY back when 7.4 first came out on a machine with BBU, and
it didn't seem to make any difference HOW I set up the hard drives,
RAID-5, 1+0, 1 it was all about the same.  With BBU the transactions per
second varied very little.  If I recall correctly, it was something like
600 or so tps with pgbench (scaling and num clients was around 20 I
believe) It's been a while.

In the end, that server ran with a pair of 18 Gig drives in a RAID-1 and
was plenty fast for what we used it for.  Due to corporate shenanigans
it was still running pgsql 7.2.x at the time.  ugh.

I've not got access to a couple of Dell servers I might be able to test
this on...  After our security audit maybe.