On 23/02/2011 15:25, John Robinson wrote:
On 23/02/2011 13:56, David Brown wrote:
[...]
Incidentally, what's your opinion on a RAID1+5 or RAID1+6 setup, where
you have a RAID5 or RAID6 build from RAID1 pairs? You get all the
rebuild benefits of RAID1 or RAID10, such as simple and fast direct
copies for rebuilds, and little performance degradation. But you also
get multiple failure redundancy from the RAID5 or RAID6. It could be
that it is excessive - that the extra redundancy is not worth the
performance cost (you still have poor small write performance).
I'd also be interested to hear what Stan and other experienced
large-array people think of RAID60. For example, elsewhere in this
thread Stan suggested using a 40-drive RAID-10 (i.e. a 20-way RAID-0
stripe over RAID-1 pairs), and I wondered how a 40-drive RAID-60 (i.e. a
10-way RAID-0 stripe over 4-way RAID-6 arrays) would perform, both in
normal and degraded situations, and whether it might be preferable since
it would avoid the single-disk-failure issue that the RAID-1 mirrors
potentially expose. My guess is that it ought to have similar random
read performance and about half the random write performance, which
might be a trade-off worth making.
Basically you are comparing a 4-drive RAID-6 to a 4-drive RAID-10. I
think the RAID-10 will be faster for streamed reads, and a lot faster
for small writes. You get improved safety in that you still have a
one-drive redundancy after a drive has failed, but you pay for it in
longer and more demanding rebuilds. But certainly RAID60 (or at least
RAID50) seems to be a choice many raid controllers support, so it must
be popular.
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