RE: Reverse of XOR.

My understanding of how RAID 5 works.
Raid 5 stores in the strip a parity bit.  I seem to recall it uses even parity, 
although it might use odd.

For sake of argument, say you have 5 disks in your array.  And say we are 
looking at the stripe on the 5th disk for the first 4.  Some semi random bit 
patterns:
Disk1  Disk2  Disk3  Disk4  Disk5
  0      0      0      0      0
  0      0      0      1      1
  0      1      1      0      0
  1      0      1      0      0
  0      1      1      1      1

Disk5 will set the bit on or off to make the parity even.  It adds up the bits 
in the other disks, if they are odd, it will set the bit to 1, if they are 
even, it will set it to 0.

Now, loosing any disk, you can determine what it's bit was supposed to be by 
the other bits, including the stripe.  In the example given, if we loose Disk2 
for the first sample, the other bits add up to 0, since it has to be even, 
Disk2 had to have been a 0.  For the last example, the other bits add up to 3, 
since it has to be even, Disk 2 had to have been a 1.

I may have it backwards, and perhaps it makes them odd instead of even, and 
this may even bee a setting in Raid, just not sure, but it doesn't really 
matter for the concept.

Regards,

Jim Langston

-----Original Message-----
From: Mark Waterbury [mailto:mark.s.waterbury@worldnet.att.net]

Hello, Walden:

RAID-5 is based on slightly different technology, using "striping"
and overlapping parity, etc., to acheive the same effects, similar
to ECC for main memory.