On Wed, Jul 11, 2012 at 08:36:41PM -0500, Jonathan Brassow wrote:
> + [raid10_copies <# copies>]
> + [raid10_format <near|far|offset>]
> + These two options are used to alter the default layout of
> + a RAID10 configuration. The number of copies is can be
> + specified, but the default is 2. There are also three
> + variations to how the copies are laid down - the default
> + is "near". Near copies are what most people think of with
> + respect to mirroring. If these options are left unspecified,
> + or 'raid10_copies 2' and/or 'raid10_format near' are given,
> + then the layouts for 2, 3 and 4 devices are:
> + 2 drives 3 drives 4 drives
> + -------- ---------- --------------
> + A1 A1 A1 A1 A2 A1 A1 A2 A2
> + A2 A2 A2 A3 A3 A3 A3 A4 A4
> + A3 A3 A4 A4 A5 A5 A5 A6 A6
> + A4 A4 A5 A6 A6 A7 A7 A8 A8
> + .. .. .. .. .. .. .. .. ..
> + The 2-device layout is equivalent 2-way RAID1. The 4-device
> + layout is what a traditional RAID10 would look like. The
> + 3-device layout is what might be called a 'RAID1E - Integrated
> + Adjacent Stripe Mirroring'.
> +
> + If 'raid10_copies 2' and 'raid10_format far', then the layouts
> + for 2, 3 and 4 devices are:
> + 2 drives 3 drives 4 drives
> + -------- -------------- --------------------
> + A1 A2 A1 A2 A3 A1 A2 A3 A4
> + A3 A4 A4 A5 A6 A5 A6 A7 A8
> + A5 A6 A7 A8 A9 A9 A10 A11 A12
> + .. .. .. .. .. .. .. .. ..
> + A2 A1 A3 A1 A2 A4 A1 A2 A3
> + A4 A3 A6 A4 A5 A8 A5 A6 A7
> + A6 A5 A9 A7 A8 A12 A9 A10 A11
The trick here for 4 drives is to keep the array running even if some 2 drives fail.
Your layout does not so so. Only one drive may fail at any time.
I think a better layout is (for 4 drives)
A1 A2 A3 A4
A5 A6 A7 A8
.................
A2 A1 A4 A3 (Swich in pairs for N=2)
A6 A5 A8 A7
Here all of the drive combinations 1+3, 1+4, 2+3, 2+4 may fail, and the array should
still be running.. 1+2 and 3+4 could not fail without destroying the array.
This would give a 66,7 % chance of the array surviving 2 disk crashes.
That is better than the 0 % that the documented scheme has.
the same scheme could go for all even numbers of N in a raid10,far layout.
consider the drives in pairs, and switch the blocks within a pair.
I think this could be generalized to N-copies: treat every group N drives,
as N copies of the same set of selection of blocks.
Then any N-1 of the disks in the group could fail and the arry still
be running. Works then for arrays with straight multipla of N disks .
I am not sure that ordinary raid10 does so, but Neil has indicated so.
I would be grateful if you could check this, and
also test what happens with your code if you have any combination of 2 drives
fail for the 4 drive case.
> +
> + If 'raid10_copies 2' and 'raid10_format offset', then the
> + layouts for 2, 3 and 4 devices are:
> + 2 drives 3 drives 4 drives
> + -------- ------------ -----------------
> + A1 A2 A1 A2 A3 A1 A2 A3 A4
> + A2 A1 A3 A1 A2 A4 A1 A2 A3
> + A3 A4 A4 A5 A6 A5 A6 A7 A8
> + A4 A3 A6 A4 A5 A8 A5 A6 A7
> + A5 A6 A7 A8 A9 A9 A10 A11 A12
> + A6 A5 A9 A7 A8 A12 A9 A10 A11
The same problem here with 2 failing drives (for the 4 drive case).
However I dont see an easy solution to this problem.
> + Here we see layouts closely akin to 'RAID1E - Integrated
> + Offset Stripe Mirroring'.
> +
> + Thanks wikipedia 'Non-standard RAID levels' for the layout
> + figures:
> + http://en.wikipedia.org/wiki/Non-standard_RAID_levels
Wikipedia may be in error wrt. the block orders.
besT regards
Keld
--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel
