On Fri, 13 Jul 2012 03:15:05 +0200 keld@xxxxxxxxxx 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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