Friday, 15 April 2011

Admin: Disk Configuration RAID 0+1

I saw an interesting question SQL Server Central recently on RAID Configurations. Essentially, it was asking which RAID level is more tolerant to multiple disk failures RAID 1+0 or RAID 0+1.

I have to be honest, i'd never even heard of RAID 0+1 before then and it got me thinking exactly what the configuration would look like. So i understand that 0 is striped and 1 is mirroed but how does mirror/stripe differ to stripe/mirror? Reading wikipedia gave me some information but to be honest, it still wasn't 100% clear so i had to get out my pencil and paper to try and visualise the difference.

I think i've got it, hopefully aided by the following diagram:

Essentially, if you have a RAID0+1 array, the whole sum of the disks is mirrored, not the individual disks (10 = 10). So if you have a disk failure, then the mirror is broken as there is no quorum disk to calculate the missing disk data and read requests can only be serviced by the one stripe set (7 != 10). This leaves you exposed to any disk failure in the remaining stripe set giving you a increased risk of data loss. In a RAID 1+0 array, as each disk is mirrored requests can be serviced by either array and as long as you don't suffer a failure of a pair of mirrored disks, your data is available.

I'm not quite sure what the benefits are of using one over the other. Once there has been a disk failure, I suppose you get better write performance with 0+1 as it is effectively a RAID0 array but i'm not sure that counts as a benefit. I'll be sticking with RAID 1+0.

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