Re: RAID5E

[Erik Mouw <erik@harddisk-recovery.com>]

On Wed, May 31, 2006 at 04:27:54PM +0200, Mattias Wadenstein wrote:
> On Wed, 31 May 2006, Bill Davidsen wrote:
> >Where I was working most recently some systems were using RAID5E (RAID5 
> >with both the parity and hot spare distributed). This seems to be highly 
> >desirable for small arrays, where spreading head motion over one more 
> >drive will improve performance, and in all cases where a rebuild to the 
> >hot spare will avoid a bottleneck on a single drive.
> >
> >Is there any plan to add this capability?
> 
> What advantage does that have over raid6? You use exactly as many drives 
> (n+2), with the disadvantage of having to do a rebuild without parity when 
> a drive fails and a raid failure at a double disk failure.

Advantage:
- Easier to calculate the checksum (RAID5 XOR instead of a generator
  polynome with RAID6)
- Higher throughput compared to standard RAID5
- Actually uses the hot spare

Disadvantage:
- Doesn't protect against double disk failures, but RAID5 also doesn't

Note that you could also do RAID6E.


Erik

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