Re: Tuning PostgreSQL - Mailing list pgsql-performance

On 2003-07-22 09:04:42 +0200, Alexander Priem wrote:
> Hi all,
>
> Vincent, You said that using RAID1, you don't have real redundancy. But
> RAID1 is mirroring, right? So if one of the two disks should fail, there
> should be no data lost, right?
>

Right. But the proposal was a single disk for WAL, without redundancy, and I
argued that wasn't really safe. RAID1 by itself is extremely safe, possibly
even the safest RAID type there is.

> I have been thinking some more. 18Gb drives are cheaper than 36 or 72Gb
> drives. I don't know if I can get the money for this, but how would the
> following setup sound?
>
> Two 18Gb (15.000rpm) disks in RAID1 array for Operating System + WAL.
> Four 18Gb (15.000rpm) disks in RAID5 array for data.
>

Our own testing has shown that a 6 disk RAID-10 array is faster than what you
describe. Of course, this is very much dependant on how much INSERT/UPDATES
you generate (which taxes your WAL more), so your mileage may vary.

> For the same amount of money, I could also get:
>
> Two 36Gb (10.000rpm) disks in RAID1 array for Operating System + WAL.
> Five/Six 36Gb (10.000rpm) disks in RAID5 array for data.
>

It is said that a higher RPM is particularly useful for a WAL disk. So you
might consider using two 18GB 15K rpm drives for a RAID-1 WAL disk (+OS and
swap), and using 36GB 10K rpm disks in a RAID-5 array if you need that
diskspace.

> Which would be the best of the above? The one with four 15k-rpm disks or the
> one with five/six 10k-rpm disks?
> Would these configs be better than all disks in one huge RAID5 array? There
> are so many possible configs with RAID.......
>

15K rpm disks are significantly faster than 10K rpm disks. If your only
concern is performance, buy 15K rpm disks. If you want more diskspace for your
money, fall back to larger 10K rpm disks.

I personally think seperate WAL disks are vastly overrated, since they haven't
shown a big performance gain in our own tests. But as I have said, this is
extremely dependant on the type of load you generate, so only your own tests
can tell you what you should do in this respect.

About RAID types: the fastest RAID type by far is RAID-10. However, this will
cost you a lot of useable diskspace, so it isn't for everyone. You need at
least 4 disks for a RAID-10 array. RAID-5 is a nice compromise if you want as
much useable diskspace as possible and still want to be redundant. RAID-1 is
very useful for small (2-disk) arrays.

If you have the time and are settled on buying 6 disks, I'd test the following
scenarios:
- 6-disk RAID-10 array (should perform best)
- 4-disk RAID-10 array containing data, 2-disk RAID-1 array for WAL, OS, etc
- 4-disk RAID-5 array containing data, 2-disk RAID-1 array for WAL, OS, etc
- 6-disk RAID-5 array (will probably perform worst)


Hope this helps.

Vincent van Leeuwen
Media Design - http://www.mediadesign.nl/