On Fri, Jul 13, 2012 at 11:27:17AM +1000, NeilBrown wrote: > 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. OK, I also think it is only sensible to reuse the code already done. I misunderstood then your mail on not to repeat mistakes - which I took to mean that Barrow should not implement things with mistakes. Maybe that means to not make hooks to MD code that is a mistake? So Barrow will implement the improved far layout once there is MD code for it, and then he can make the neceessary hooks in DM 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. On further investigations I agree that you can survive more than one drive failing with the current layout. > > > 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). Well, I have an idea for the odd number of devices: Have the disks arranged in groups (for N=2 in pairs) and then the last group extended with the leftover disks in the way it is done now. For 2 copies, this would be a number of pairs, and then a rest group of 3 disks. For 3 copies, this would be a number of triplets, and then 4 or 5 disks in the last group. Can I assume, Neil, that you agree with the rest I wrote? :-) Especially that we should only advice the new layout, and there is no reason for the current implementation except for backwards compatibility? best regards keld -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
