[CentOS] RAID 6 - opinions

Fri Apr 12 02:55:51 UTC 2013
Keith Keller <kkeller at wombat.san-francisco.ca.us>

On 2013-04-12, Miranda Hawarden-Ogata <hawarden at ifa.hawaii.edu> wrote:
> RAID6 means you can handle 2 disk failures, but the third one will drop 
> your array, if I'm remembering correctly. And the larger the number of 
> disks, the higher the chance that you'll have disk failures...

Yes, and yes.  But different configurations of other RAID levels will
give you different levels of protection--not "better" or "worse",
because that needs to be evaluated in context.

For example, as has been noted, RAID6 can lose up to two drives, and the
third lost drive loses the array [0].  A 12-drive RAID10, with six
two-drive RAID1 components, can lose up to six drives, but only the
right six drives--losing both drives of one RAID1 loses the entire
array.  On the other side of things, rebuilding a 12-drive RAID6 will
take much longer than rebuilding one RAID1 component of a RAID10.  And
as one more example, a 12-drive RAID50, with three four-drive RAID5
components, can lose up to three drives, one from each component, but
two drives from one RAID5 loses the array.  Rebuild times will be longer
than RAID10 but shorter than RAID6.  (There are also performance
questions, which I know little about.)

RAID6 is certainly the most efficient way, space-wise, to allocate
drives such that you can lose up to two drives before losing the array.
So if maximizing storage space is the primary concern, greater than
performance, RAID6 is likely the best choice.  But, as is often repeated
here, on the md RAID list, and elsewhere, ***RAID IS NOT A BACKUP
SOLUTION!!!***  If you care about your data you need to back it up
elsewhere.  Do *not* rely solely on RAID to keep your data safe!  All
sorts of bad things can happen: a flaky controller can cause filesystem
problems, and a badly defective controller can completely destroy the
array.  RAID allows you to tolerate some failure, but it can't save your
data from catastrophe.


[0] "loses the array" here means that it won't be mountable without
some sort of expensive drive recovery process.

kkeller at wombat.san-francisco.ca.us