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Re: With 4 disks should I go for RAID 5 or RAID 10

From: "Fernando Hevia" <fhevia(at)ip-tel(dot)com(dot)ar>
To: "'pgsql-performance'" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: With 4 disks should I go for RAID 5 or RAID 10
Date: 2007-12-26 20:52:27
Message-ID: 0f9701c84801$399dcff0$ (view raw, whole thread or download thread mbox)
Lists: pgsql-performance

> David Lang Wrote:
> with only four drives the space difference between raid 1+0 and raid 5
> isn't that much, but when you do a write you must write to two drives (the
> drive holding the data you are changing, and the drive that holds the
> parity data for that stripe, possibly needing to read the old parity data
> first, resulting in stalling for seek/read/calculate/seek/write since
> the drive moves on after the read), when you read you must read _all_
> drives in the set to check the data integrity.

Thanks for the explanation David. It's good to know not only what but also
why. Still I wonder why reads do hit all drives. Shouldn't only 2 disks be
read: the one with the data and the parity disk?

> for seek heavy workloads (which almost every database application is) the
> extra seeks involved can be murder on your performance. if your workload
> is large sequential reads/writes, and you can let the OS buffer things for
> you, the performance of raid 5 is much better.

Well, actually most of my application involves large sequential
reads/writes. The memory available for buffering (4GB) isn't bad either, at
least for my scenario. On the other hand I have got such strong posts
against RAID 5 that I doubt to even consider it.

> Linux software raid can do more then two disks in a mirror, so you may be
> able to get the added protection with raid 1 sets (again, probably not
> relavent to four drives), although there were bugs in this within the last
> six months or so, so you need to be sure your kernel is new enough to have
> the fix.

Well, here rises another doubt. Should I go for a single RAID 1+0 storing OS
+ Data + WAL files or will I be better off with two RAID 1 separating data
from OS + Wal files?

> now, if you can afford solid-state drives which don't have noticable seek
> times, things are completely different ;-)

Ha, sadly budget is very tight. :)


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