On Thursday 15 July 2004, valen at tuatha.org (John P. Looney) wrote:
> It's not that magic; it's just RAID4. When you keep parity on just one
>disk, it's reasonably easy to just keep adding new disks, as long as the
>parity disk has sufficent storage. Of course, as the parity disk gets
>written to all the time, it has to be a lot faster.
I understood that WAFL was RAID-4 based. What I don't understand is how adding
a disk is handled. Take a 3 disk RAID-4 - a block of data is striped across
two disks A & B, and parity is on the parity disk P. So to store 2 blocks of
data, you use 1 block of disk A, 1 of disk B and 1 of disk P.
If instead of 2 data disks you have three, then to store 3 blocks of data, you
use 1 block of disk A, 1 of disk B, 1 of disk C, and 1 of disk P.
What puzzles me is how you can transparently migrate from the AB case to the
ABC case. There'd have to be some information stored as to how parity data has
been allocated etc. This is the magic part to me.
Niall
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