On 2011/09/21 06:05 AM, NeilBrown wrote:
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?
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
That makes sense then that there's no point in a hot spare when you're
using RAID5 - as you say, rather use RAID6. This concept could be used
on top of a RAID6 too. However, I also see what you mean by the
complexity issue.
There's diminishing benefits when you have lots of disks and, with fewer
disks, you're better off having a bigger RAID6. Given a typical RAID6 (8
disks-worth of capacity), the potential improvements are still
*relatively* significant, but much less than the numbers given
previously. Maximum potential numbers:
read speed improvement: 9.09%
sync progress at disk fail-time before any pro-active sync work starts:
12.5%
In my books, any trickery that improves integrity is a good thing but,
until I have time to hack around in mdadm code (not likely at the
moment), I guess I'll leave it to whoever else who thinks these numbers
are worth it. :)
Thanks, all.
--
Brendan Hide
http://swiftspirit.co.za/
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