[ILUG] Why RAID

[Niall O Broin]

On Thursday 15 July 2004, valen at tuatha.org (John P. Looney) wrote:

>On Thu, Jul 15, 2004 at 10:10:14AM +0100, Colm Buckley mentioned:
>> When you add the new (C) drive, all of its data are zeroes, which 
>> doesn't change the parity data (N XOR 0 = N).  Fiendishly clever.
>
> I knew that once. Wow. That's so cool.

It still puzzles me though, I'm sorry to say. Blocks of data added before the
new drive has been added are striped across 2 disks, but afterwards, they're
striped across 3 disks. How does the controller keep track of this? And if
you're adding an extra drive because the original 2 were, for arguments sake,
100% full, how does striping take place in future, as there's space only on
one drive ? And what about parity ? Do you have to have a big parity drive
from the get go (I think someone mentioned that earlier)



Niall