Re: Advice on raid/lvm
On Wed, Apr 08, 2009 at 06:34:09PM -0500, Mark Allums (mark@allums.com) wrote:
> Not really answering your question directly, but may I suggest, if cost
> is not *absolutely* critical, that you consider RAID 10? If it is a
> server, then certainly you will want to get away from a three-drive RAID
> 5. A RAID 10 is a good compromise between redundancy, speed, and cost.
> It just takes four drives instead of three (or two.)
Whether it's a good compromise depends on how much speed vs. reliability
matter. A 4-disk RAID10 can survive 2-disk failure with 2/3 probability
(RAID 01 only with 1/3), whereas RAID6 can handle loss of any two disks
but is also much slower. With RAID5 the compromise is between capacity and
reliability, and as noted, the balance keeps getting worse as disks grow.
It may still make sense in some scenarios though.
But simply two separate RAID1 instances (which I understand OP was
planning) is actually more robust than RAID10 (in that even loss
of three disks won't lose _all_ data in there), and it can be done
with disks of different sizes. So I'd prefer that if speed is not
critical but reliability requirements don't quite warrant RAID6 either.
Of course it would also be possible to divide the 1TB disks in half,
use one half as RAID1 and the other as RAID10 together with the
500GB disks for speed, but if the latter are slower to begin with,
the speed gain may not be all that great.
So, I think OP's original plan (as I understood it) is a sound
compromise between cost, reliability, speed and simplicity
under a wide range of requirements.
--
Tapani Tarvainen
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