[Gllug] RAID 1+0 vs 0+1

[Christian Smith]

On Thu, 9 Mar 2006, Rich Walker wrote:

>Richard Huxton <dev at archonet.com> writes:
>
>>
>> Hmm, I'd expect better performance on one disk failure with 1+0
>> too. You've got 5 spindles still running versus 3 for 0+1, and a 2/3
>> chance of reading from a mirrored disk.
>
>AIUI, most significant array failures happen not when one disk goes
>while the system is running; they happen when you shut down a server
>that's been on 24x7 for the last year or so, and you get *multiple*
>failures on power-up.
>
>There's probably some neat statistical analysis that can be done to
>prove the point, but:


Assuming you have 2 disks of the 6 fail, the stripe then mirror will be
broken if:
  disk 1 and disk 2 are in different stripes =
    1/6 + 3/5 = 23 / 30 = 77%

The mirror plus strip will be broken if:
   disk 1 and disk 2 are in the same mirror:
    1/6 + 1/5 = 11 / 30 = 37%

So, given this analysis, mirror then strip is significantly safer.

Someone check my maths!

Of course, if you had 6 disks, then surely you would use RAID 6, which can
tolerate any 2 disk failures and still have 2/3 disk utilisation:)


Christian

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