Thanks for the suggestion. The documentation is correct, as far as I can tell. What you take issue with is that a higher level of redundancy can be achieved by laying down the copies differently. Neil touched on that in this message: http://marc.info/?l=linux-raid&m=134136516029779&w=2 When it is available to MD, I'll make it available to dm-raid also. brassow On Jul 12, 2012, at 11:22 AM, keld@xxxxxxxxxx wrote: > 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
