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
--
__________
Brendan Hide
Mobile: brendan.cell@xxxxxxxxxxxxxxxxx (plain text only please)
Web Africa - Internet Business Solutions
http://www.webafrica.co.za/?AFF1E97
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