Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

On Fri, Nov 22, 2002 at 08:52:48AM -0500, Tom Lane wrote:
> Mario Weilguni <mweilguni@sime.com> writes:
> > If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
>
> Ideally there should be *nothing* on the WAL drive except WAL; you don't
> ever want that disk head seeking away from the WAL.  Put the temp files
> on the data disk.

Unless the interface and disks are so fast that it makes no
difference.

Try as I might, I can't make WAL go any faster on its own controller
and disks than if I leave it on the same filesystem as everything
else, on our production arrays.  We use Sun A5200s, and I have tried
it set up with the WAL on separate disks on the box, and on separate
disks in the array, and even on separate disks on a separate
controller in the array (I've never tried it with two arrays, but I
don't have infinite money, either).  I have never managed to
demonstrate a throughput difference outside the margin of error of my
tests.  One arrangement -- putting the WAL on a separate pair of UFS
disks using RAID 1, but not on the fibre channel -- was demonstrably
slower than leaving the WAL in the data area.

Nothing is proved by this, of course, except that if you're going to
use high-performance hardware, you have to tune and test over and
over again.  I was truly surprised that a separate pair of VxFS
RAID-1 disks in the array were no faster, but I guess it makes sense:
the array is just as busy in either case, and the disks are really
fast.  I still don't really believe it, though.

A

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