large RAID volume partition strategy

[Matthew Seaman]

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Clayton Milos wrote:

> If you want awesome performance and reliability the real way to go is
> RAID10 (or more correctly RAID 0+1).

RAID10 and RAID0+1 are very different beasts.  RAID10 is the best
choice for a read/write intensive f/s with valuable data, exactly
what you need to support a RDBMS.  It is built by pairing up all of
the drives as RAID1 mirrors[*] and then creating a RAID0 stripe
across all of the mirrors.  It's the least economical RAID setup,
giving you a usable space which is 50% of the total raw disk space,
but it is the most resilient -- potentially being able to survive
half of the drives failing -- and much the best performing of the
RAID types.

RAID0+1 on the other hand is what you give to someone you don't like
very much.  In this case, you divide the disks into two equal sets,
create a RAID0 stripe over each set and then a RAID1 mirror over the
stripes.  It has the /delightful/ feature that failure of any one
drive immediately puts half of the available disks out of action: ie
it is *less* resilient than any other RAID setup (other than a RAID0
stripe over all the drives).  Space economy-wise it's exactly like
RAID10 and performance characteristics are pretty similar to RAID10,
leading to the obvious conclusion: use RAID10 instead.

	Cheers,

	Matthew

[*] The correctly paranoid sysadmin will of course ensure that each
of the disks in those pairs hangs off a different bus, comes from a
different manufacturing batch and is preferably connected to a
different controller and with different, independent power supplies.
 Or, in extreme cases, that each half of the mirrors are in
completely different datacenters.

- --
Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW
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