Re: making a hot spare ... hot

[NeilBrown <neilb@suse.de>]

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On Wed, 21 Sep 2011 05:30:17 +0200 Brendan Hide <brendan@swiftspirit.co.za>
wrote:

> Hi all
> 
> To the point: When a disk is designated as a hot spare, would it be of 
> benefit to spread copies of data chunks from the other disks onto the 
> hot spare even before a failure? Has this been tried before?
> 
> If its not already being done, it'd have a small positive consequence 
> for performance as well as data integrity, with relatively little to no 
> negative consequences. Benefits would diminish the larger the array, 
> much like the performance difference between raid3 and raid5. Read 
> speeds would theoretically increase and write speeds should not decrease 
> except in the case of poor hardware.
> 
> Given a 6-disk raid5 (5 "data" disks + 1 spare) array, a re-sync will 
> start at 25% progress from the moment a disk gets dropped out of the 
> array. The theoretical max read speed will also increase by 16% by 
> reading from 6 disks instead of 5. The cons will be that, when writing, 
> an extra write will need to occur to the "spare" disk. Though this 
> shouldn't have any performance penalties on modern hardware I can still 
> see it as being a concern.
> 
> I suspect something like this might have been suggested before - but I 
> haven't been able to find any reference to something along these lines 
> online. I'll welcome any discussion or links to relevant information.
> 
> Thanks.
> 
> Key:
> 0-F: Data Chunks
> P: Parity
> 
> Layout of standard RAID5 + 1 standard spare
> 
> Disk0: 048C
> Disk1: 159P
> Disk2: 26PD
> Disk3: 3PAE
> Disk4: P7BF
> Disk5: Spare (empty)
> 
> Chunks read per read "cycle": 5
> Time to read all 16 data chunks: 4 cycles
> 
> Layout of standard RAID5 + 1 "hot" spare:
> Disk0: 048C
> Disk1: 159P
> Disk2: 26PD
> Disk3: 3PAE
> Disk4: P7BF
> Disk5: 05AF
> 
> Chunks read per "cycle": 6
> Time to read all 16 data chunks: 3 cycles
> 

I see what you are getting at, but I doubt the value justifies the extra
complexity.
If you want more redundancy and have a spare device - use RAID6.

NeilBrown


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