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"Frank van Bortel" <fvanbortel_at_netscape.net> wrote in message
news:cprmib$q43$1_at_news1.zwoll1.ov.home.nl...
> Jonathan Lewis wrote:
> > Sorry about the delay in replying. Two weeks
> > out of the country, and lots to catch up on.
> >
> > I rarely get the numbers right, but a fixed parity
> > disk is either RAID-3 or RAID-4. When you
> > move to RAID-5, I believe the parity is described
> > as 'block-level rotating parity', so every disc gets
> > a fair share.
> >
> > A pattern across five disc would look something
> > like the following (where D = data and P = parity
> > block).
> >
> > D D D D P
> > D D D P D
> > D D P D D
> > D P D D D
> > P D D D D
> >
>
> RAID-3 as well as 4 have a fixed parity disk.
> You are correct about RAID-5.
>
> In addition to the redundancy in data, the Hamming codes
> used for parity calculations, can recreate the missing
> parity bit from remaining disks, thus allowing one disk
> to go bad.
> However, 2 bits cannot be recalculated
> (aka when 2 disks go bad: failure).
>
I've never seen it, but read that someone offers a RAID 6. It doubles the parity data, so that recovery can be done in the event of two drives in an array failing at the same time. Received on Fri Dec 17 2004 - 09:35:15 CST