Date: Sat, 28 Jun 2008 19:01:13 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: prad <prad@towardsfreedom.com> Cc: freebsd-questions@freebsd.org Subject: Re: first pre-emptive raid Message-ID: <48667C69.2060109@infracaninophile.co.uk> In-Reply-To: <20080628095613.38e92182@gom.home> References: <20080628005702.2137bb8c@gom.home> <200806281302.20814.pieter@degoeje.nl> <20080628095613.38e92182@gom.home>
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prad wrote:
> why in diagram 20-3 of the handbook do they show 2 parity blocks though=
> for disk3 and disk4? why would you ever have more than 1 for any single=
> disk?
The diagram shows a RAID5 made out of a number of disk stripes spread
across 4 physical drives. It's the /stripes/ that are the basic building=
block, not the disks in that implementation. What RAID5 does is take
N-1 data blocks and adds a Nth parity block as a stripe across the N
drives. ie. reading across the diagram the first stripe consists of the
top row of 3 data blocks on the first three disks plus the 1 parity block=
on the top row on the 4th disk. The next stripe (the 2nd row down) arrang=
es=20
it as data data parity data, and so forth for the succeeding stripes.
The parity blocks are staggered across the drives to even out the traffic=
=20
levels on each spindle. As the diagram shows, it's not always possible to=
=20
have an exactly equal balance of parity and data blocks per drive. The
diagram is simplistic though -- in a real system the stripe size would be=
something like N*128k -- which means that there would be millions of stri=
pes
on typical GB sized disks, so the differences between drives are entirely=
=20
neglible.
You have to have at least three disks to make a RAID5, and while=20
theoretically you can have as many disks as you want, there's a practical=
limit somewhere around 12-13 drives [Remember, you have to calculate the
XOR of all the data blocks in the stripe on any IO operation, and with so=
=20
many drives, that rapidly becomes onerous.] The sweet spot seems to be
around 7 drives.
There is a variant not supported by vinum called RAID6 -- this is simply
RAID5 with the parity block doubled up as a backup. It's relatively new
and available under FreeBSD only by using hardware cards (Areca, 3ware et=
c.)
All this does is allow the file system to survive loss of any *two* disks=
--=20
something that is more likely to occur than you might think, particularly=
as the number of disks per RAID goes up.
Cheers,
Matthew
--=20
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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