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

[NeilBrown <neilb@suse.de>]

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On Fri, 13 Jul 2012 03:15:05 +0200 keld@keldix.com wrote:

> I think the layout you described should not be promoted at all,
> and only kept for backward compatibility. As there is no backward 
> compatibility in your case I think it is an error to implement it.
> I understand that you do not reuse any of the MD code here?

Not correct.  The whole point of this exercise is to reuse md code.


> The flaw is worse than Neil described, as far as I understand.
> With n=2 you can in the current implementation only have 1 disk failing,
> for any numbers of drives in the array. With the suggested layout
> then for 4 drives you have the probability of surviving 66 % 
> of 2 drives failing. This get even better for 6, 8 .. disks in the array.
> And you may even survive 3 or more disk failures, dependent on the number
> of drives employed. The probability is the same as  for raid-1+0

Also not correct.  You can certainly have more than one failed device
providing you don't have 'n' adjacent devices all failed.
So e.g. if you have 2 drives in a far-2 layout then you can survive the
failure of three devices if they are 0,2,4 or 1,3,5.


> 
> > When it is available to MD, I'll make it available to dm-raid also.
> 
> Please dont implement it in the flawed  way. It will just create a number of problems
> for when to switch over and convert between the two formats, and then which should
> be the default (I fear some would say the old flawed should be the default), and we need
> to explain the two formats and implement two sets of repairs and so on.

This "flawed" arrangement is the only one that makes sense for an odd number
of devices (assuming 2 copies).

NeilBrown

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