[Gllug] RAID 1+0 vs 0+1

[Andy Smith]

On Thu, Mar 09, 2006 at 03:12:46PM +0000, Steve Nelson wrote:
> A debate arose on irc as to whether there's a difference between RAID
> 1+0 and 0+1.

A difference in terms of what?

> I contend that 1+0 is more resilient.  Our esteemed Mr
> Brooks disagreed, and stated "There is no difference".

In terms of resilience there is no difference.  You can stand to
lose up to 3 disks in either setup.  If you have a mirror of two
stripes then you can afford to lose one side of the mirror, which is
a complete 3-disk stripe.  If you have a stripe of 3 mirrors then
you can afford to lose one half of each mirror, which is 3 disks.

However 1+0 (mirror then stripe) is usually preferred because on any
disk failure you only have to rebuild that failed disk from its
mirror.  With 0+1, any disk failure takes a whole stripe out of
commission and degrades the single big mirror, and the mirror stays
degraded until you've replaced all faulty disks and rebuilt it all.

RAID-10 (1+0) will spend less time running degraded when disk
failures do happen.

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