Re: PowerEdge 2950 questions - Mailing list pgsql-performance

On Tue, 2006-08-22 at 17:56 -0400, Bucky Jordan wrote:
> Hi Jeff,
>
> My experience with the 2950 seemed to indicate that RAID10x6 disks did
> not perform as well as RAID5x6. I believe I posted some numbers to
> illustrate this in the post you mentioned.
>

Very interesting. I always hear that people avoid RAID 5 on database
servers, but I suppose it always depends. Is the parity calculation
something that may increase commit latency vs. a RAID 10? That's
normally the explanation that I get.

> If I remember correctly, the numbers were pretty close, but I was
> expecting RAID10 to significantly beat RAID5. However, with 6 disks,
> RAID5 starts performing a little better, and it also has good storage
> utilization (i.e. you're only loosing 1 disk's worth of storage, so with
> 6 drives, you still have 83% - 5/6 - of your storage available, as
> opposed to 50% with RAID10).

Right, RAID 5 is certainly tempting since I get so much more storage.

> Keep in mind that with 6 disks, theoretically (your mileage may vary by
> raid controller implementation) you have more fault tolerance with
> RAID10 than with RAID5.

I'll also have the Slony system, so I think my degree of safety is still
quite high with RAID-5.

> Also, I don't think there's a lot of performance gain to going with the
> 15k drives over the 10k. Even dell only says a 10% boost. I've
> benchmarked a single drive configuration, 10k vs 15k rpm, and yes, the
> 15k had substantially better seek times, but raw io isn't much
> different, so again, it depends on your application's needs.

Do you think the seek time may affect transaction commit time though,
rather than just throughput? Or does it not make much difference since
we have writeback?

> Lastly, re your question on putting the WAL on the RAID10- I currently
> have the box setup as RAID5x6 with the WAL and PGDATA all on the same
> raidset. I haven't had the chance to do extensive tests, but from
> previous readings, I gather that if you have write-back enabled on the
> RAID, it should be ok (which it is in my case).

Ok, I won't worry about that then.

> As to how this compares with an Opteron system, if someone has some
> pgbench (or other test) suggestions and a box to compare with, I'd be
> happy to run the same on the 2950. (The 2950 is a 2-cpu dual core 3.0
> ghz box, 8GB ram with 6 disks, running FreeBSD 6.1 amd64 RELEASE if
> you're interested in picking a "fair" opteron equivalent ;)
>

Based on your results, I think the Intels should be fine. Does each of
the cores have independent access to memory (therefore making memory
access more parallel)?

Thanks very much for the information!

Regards,
    Jeff Davis