ZFS or UFS for 4TB hardware RAID6?

[Matthew Seaman]

Richard Mahlerwein wrote:
 
> With 4 drives, you could get much, much higher performance out of
> RAID10 (which is alternatively called RAID0+1 or RAID1+0 depending on
> the manufacturer

Uh -- no.  RAID10 and RAID0+1 are superficially similar but quite different
things.  The main differentiator is resilience to disk failure. RAID10 takes
the raw disks in pairs, creates a mirror across each pair, and then stripes
across all the sets of mirrors.  RAID0+1 divides the raw disks into two equal
sets, constructs stripes across each set of disks, and then mirrors the
two stripes.

Read/Write performance is similar in either case: both perform well for 
the sort of small randomly distributed IO operations you'ld get when eg.
running a RDBMS.  However, consider what happens if you get a disk failure.
In the RAID10 case *one* of your N/2 mirrors is degraded but the other N-1
drives in the array operate as normal.  In the RAID0+1 case, one of the
2 stripes is immediately out of action and the whole IO load is carried by
the N/2 drives in the other stripe.

Now consider what happens if a second drive should fail.  In the RAID10
case, you're still up and running so long as the failed drive is one of
the N-2 disks that aren't the mirror pair of the 1st failed drive.
In the RAID0+1 case, you're out of action if the 2nd disk to fail is one
of the N/2 drives from the working stripe.  Or in other words, if two
random disks fail in a RAID10, chances are the RAID will still work.  If
two arbitrarily selected disks fail in a RAID0+1 chances are basically
even that the whole RAID is out of action[*].

I don't think I've ever seen a manufacturer say RAID1+0 instead of RAID10,
but I suppose all things are possible.  My impression was that the 0+1 
terminology was specifically invented to make it more visually distinctive
-- ie to prevent confusion between '01' and '10'.

	Cheers,

	Matthew

[*] Astute students of probability will point out that this really only
makes a difference for N > 4, and for N=4 chances are evens either way 
that failure of two drives would take out the RAID.

-- 
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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