On 23/02/2011 21:59, Stan Hoeppner wrote:
John Robinson put forth on 2/23/2011 8:25 AM:
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),
Actually, that's not what I mentioned.
Yes, it's precisely what you mentioned in this post:
http://marc.info/?l=linux-raid&m=129777295601681&w=2
[...]
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
[...]
First off what you describe here is not a RAID60. RAID60 is defined as
a stripe across _two_ RAID6 arrays--not 10 arrays. RAID50 is the same
but with RAID5 arrays. What you're describing is simply a custom nested
RAID, much like what I mentioned above.
In the same way that RAID10 is not specified as a stripe across two
RAID1 arrays, RAID60 is not specified as a stripe across two arrays. But
yes, it's a nested RAID, in the same way that you have repeatedly
insisted that RAID10 is nested RAID0 over RAID1.
Anyway, you'd be better off striping 13 three-disk mirror sets with a
spare drive making up the 40. This covers the double drive failure
during rebuild (a non issue in my book for RAID1/10), and suffers zero
read or write performance, except possibly LVM striping overhead in the
event you have to use LVM to create the stripe. I'm not familiar enough
with mdadm to know if you can do this nested setup all in mdadm.
Yes of course you can. (You can use md RAID10 with layout n3 or do it
the long way round with multiple RAID1s and a RAID0.) But in order to
get the 20TB of storage you'd need 60 drives. That's why for the sake of
slightly better storage and energy efficiency I'd be interested in how a
RAID 6+0 (if you prefer) in the arrangement I suggested would perform
compared to a RAID 10.
I'm positing this arrangement specifically to cope with the almost
inevitable URE when trying to recover an array. You dismissed it above
as a non-issue but in another post you linked to the zdnet article on
"why RAID5 stops working in 2009", and as far as I'm concerned much the
same applies to RAID1 pairs. UREs are now a fact of life. When they do
occur the drives aren't necessarily even operating outside their specs:
it's 1 in 10^14 or 10^15 bits, so read a lot more than that (as you will
on a busy drive) and they're going to happen.
Cheers,
John.
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