[CentOS] How to setup RAID 5 + 1 hot spare?

Thu Jan 24 16:58:22 UTC 2013
zGreenfelder <zgreenfelder at gmail.com>

I'm not really sure I can clarify things for you, but I'll give it a shot.

> I read this discussion on serverfault.com
> http://serverfault.com/questions/106131/raid-5-with-hot-spare-or-raid-10-with-no-hot-spare
> but still quite confused as some people think RAID-10 is way safer while
> some people wrote:

I'm not sure if it's so much 'way safer', as 'very nearly as safe and
provides much better performance'

> "
> I'd have to disagree with CHopper3. Since there are only 4 drives in
> this situation your failure capabilities are the same (2 drives) with
> either scenario, except with raid 10 if you happen to lose the wrong 2
> drives then you'll have a real problem. Also there is definitely an
> added benefit of having a global spare for your other RAIDs as well.
> "

this is playing a bit with probability; e.g. the chance of rolling a
die 3 consecutive times such that each roll ends up on 6 is 1/216,
while the probability of a single roll ending in a 6 (regardless of
how many previous rolls have happened) is 1/6

if the probability of a drive failure is 1/x, the probability of 2
drive failures is 1/(x^2); if you're using raid10 (mirrored sets
striped... you need 2 drives to get a mirrored set, 2 sets to be able
to stripe -> 2*2 == 4(minimum)) if that probability hits on a raid10
device, and further both drives in one of the mirror sets are the ones
to go, bad things happen.

if the same situation happens when the disks are configured to a raid5
set with a spare, you get a 1/4 chance that the second failure happens
on the spare drive and your raid5 device is able to keep going.   if
you're in a situation where you have other raid devices in the same
box and the spare can be moved to other raid sets, there's a bonus

now the downside, raid5 is always going to be slower than mirrored
stripes because the parity has to be calculated either by the OS or by
some controller somewhere.

> My new question is:
> As RAID 10 requires at least 4 hard drives (not very sure about this
> information), with only 4 hard disks, is RAID-10 without hot spare a
> better solution than RAID-5 + 1 hot spare? Why or why not?

in some ways sorta comes down to personal preference.  if you can live
with the slower performance of the raid5 and have use for the spare
(even better if you can apply it to other raid sets in the same
device), that's probably best.   if you have to have the performance,
raid10 is probably going to be better for you.

there's an old graphic I always liked.  something along the lines of

                         chose your own raid: fast, cheap, fault tolerant.
                                 you can pick 2.

hope this helps.
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