# Raid 3 over network

docwhat@gerf.org ;The Doctor What ("docwhat@gerf.org)
Fri, 02 May 1997 11:59:45 -0700

Here is what I remember:

(Any gurus out there, feel free to jump in)

BTW: This is level 3 RAID... a brief overview is located here (no
technical stuff, unfortunately):
http://www.invincible.com/rguide.htm

The smallest case (3 disks):

Disk 0 Disk 1 Disk 2
0 0 0
0 1 1
1 0 1
1 1 0

To map this to a flat storage space, you would read across the disks:
Disk 0, Disk 1, Disk 2, Disk 1 (row 2), Disk 3 (row 2), etc.

Disk 2 is an EVEN/ODD bit (which is XOR in this case). If the sum of
the bits for disk 0 and 1 are even, put in a 0. If odd, put in a 1.

If you loose ONE disk, you can recreate the information on the fly.
example, we loose disk 1 (or perhaps, since you're doing networking,
the delay talking to disk 1 is too high):

Assuming a and c are known, and x isn't:
a XOR x = c
is the same as:
a XOR c = x

So, as you can see, you can figure out ANY single missing bit, if you
have the matching bits from the rest of the disks.

You can expand this to multiple disks (i.e., you can have 6 disks, 1 of
which is the parity disk), but you'd only be able to suffer the loss of
a single disk.

NOW, expanding this to multiple disks....

Uhh.....I just got really busy.... If you need more help, please email,
sorry.

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