RE: Storage EMC

The text below reads to me that 1+0 is more fault tolerant as it is statistically less likely to fail, which agrees with what I thought and have repeatedly encountered over the last few years.

IMHO -- Mark D Powell --

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Paul Baumgartel Sent: Thursday, June 09, 2005 1:14 PM
To: oracle-l_at_freelists.org
Subject: Re: Storage EMC

Jared,

I always thought the opposite, that RAID 1+0 is more fault-tolerant, based on a paper I downloaded. Excerpt:

In either case (0+1 or 1+0), the loss of a single drive does not result in failure of the RAID system. The difference comes in the chance that the loss of a second drive from the system will result in the failure of the whole system. In RAID 0+1, you have to lose one drive from each disk set to result in the failure of the whole system.  In RAID 1+0, you have to lose all drives in a mirror.

Mathematically, the difference is that the chance of system failure with two drive failures in a RAID 0+1 system with two sets of drives is n/(2n-2) where n is the total number of drives in the system. The chance of system failure in a RAID 1+0 system with two drives per mirror is 1/(n-1). So, using the 8 drive systems shown in the diagrams [not included here, sorry], the chance that losing two drives would bring down the RAID system is 4/7 with a RAID 0+1 system and 1/7 with a RAID 1+0 system.

Paul Baumgartel

On 6/9/05, Jared Still <jkstill_at_gmail.com> wrote:
> RAID 0+1 is more fault tolerant than RAID 1+0.

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