It's the same as if I had lost two disks from the array, which RAID 6 can recover. The difference is that for each stripe (right terminology?) of the array, the overwrites didn't occur on the same two disks due to the "Q A B C P" layout rotating from stripe to stripe (each 64KB * 5 in my case). Therefore, for any given stripe, which disks were overwritten is a function of the current incorrect ordering of the disks in the array and the offset of the stripe (which tells which rotation of "Q A B C P" is used). Including knowledge of the original correct ordering of the disks then allows mapping to which two components of the original data have been lost (ie. two of Q, A, B, C, or P). Then it is a simple matter of recalculating the missing components from the survived components. I'm now almost done with an nbd-based translator that does this reversal on the fly (ie. takes /dev/sdg[a-e]5 as input and presents the translated versions as /dev/nbd[0-4]). Since I wanted to do the remapping only once per stripe, I haven't used the nbd server and client code as I'd have to run an instance of each for each of the 5 disks in my array. I've got my own server and client code that interfaces with the nbd kernel module in the same way, but provides access to all disks through the one instance. I don't want to risk messing with any of the MD or LVM metadata on the Synology, so once the translator is finished I will try assembling an array from these devices across the network on another machine and check/mount the filesystem. I was hoping that there might be a tried and tested tool already available for this, but if not I'll continue with mine. Steven On Sat, Jul 18, 2015 at 9:43 PM, Mikael Abrahamsson <swmike@xxxxxxxxx> wrote: > On Sat, 18 Jul 2015, Steven Reddie wrote: > >> I need to reverse these disk writes across then entire length of the >> array, after which I can reassemble the array in the correct order. > > > If you did not use --assume-clean, and had the wrong parity order and the > array was "resynced", I don't see how you did not irrevocably overwrote > important information and you now can't reverse this. > > -- > Mikael Abrahamsson email: swmike@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
