On Wed, 21 Sep 2011 05:30:17 +0200 Brendan Hide <brendan@xxxxxxxxxxxxxxxxx> 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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