[ILUG] Why RAID

[Paul Jakma]

On Mon, 19 Jul 2004, Des Keane wrote:

> RAID 1+0, Paul - if you have 8 disks, RAID 0+1 means 4 striped 
> mirrored to 4 striped. If disk 1 fails you're okay, you read/write 
> off 4 remaining disks (5-8). If disk 2 then fails you're stuffed. 
> RAID 1+0 means as John said disks 1+2, 3+4, 5+6, 7+8 individually 
> mirrored, then striped together. If disks 1, 3, 5, 7 all fail I'm 
> still in business. Is that not more resilient? :-)

Ah, you and john are right, yes. apologies. 1 over 0 is unlikely to 
stand a n-2 failure, 0 over 1 does stand a reasonable chance.

1 over 0, probability of second failure taking out array is 
(n-2)/(n-1), (ie 0.6.. for 4 disk, 0.85 for 8 disk, 0.88 for 10, and 
gets worse as n is increased), but 0+1 probabilty of second failure 
killing the whole array is 1/(n-1), and hence actually improves the 
more disks you add. (but the more disks, the lower the interval 
between failures of individual disks possibly).

Apologies, I was dangerously wrong. (not new for me).

> Hardware RAID controllers can have 2 characteristics:

> a replacement and get my (College's!) money back. As you all know, 
> cheaper not always = better.

and more expensive not always == better either ;)

regards,
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