[ILUG] Why RAID

[Colm Buckley]

On 15 Jul 2004, at 10:02, Niall O Broin wrote:

> 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.

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.

	Colm

-- 
Colm Buckley / colm at tuatha.org / www.colm.buckley.name / +353 87 2469146