Re: [PATCH v2] DM RAID: Add support for MD RAID10

[keld@keldix.com]

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