On 04/23/2017 05:42 PM, Andreas Klauer wrote: > On Sun, Apr 23, 2017 at 05:11:08PM +0200, Patrik Dahlström wrote: >> * I try to find 0x53EF (ext4 magic) within the first 128M of >> /dev/sd[abcde]. Not after? This will be an indication of my "New >> Offset". I need to adjust the offset a bit since the ext4 magic is >> located at 0x438 offset. > > Yes, that should be about it. > >> * I create a 5 and a 6 drive raid set and try to find an offset where >> they both carry the same raw data. With some overlays, I should be able >> to create both these raids at the same time, correct? > > Yes, two sets of overlays. > > So overlay A is your 5 disk raid5, overlay B is your 6 disk raid5, > and then you'll just have to take a stab at it with hexdump. > > So kind of like, > > hexdump --skip $((17*1024*1024*1024)) --length 4096 /dev/md42 > hexdump --skip $((17*1024*1024*1024)) --length 4096 /dev/md43 > > If that produces the same random-looking data then your reshape > might have progressed 17G-ish and you could try to use that as > a starting point for a linear device mapping that uses the > first 17G of the 6 disk raid and everything else from the 5disk. > > The further the grow processed the larger a zone of overlap there > should be since more data ends up on the additional drive so > the original representation of the same data isn't yet written > into on the old drives. > > Does that make sense? > > 5 disks: a b c d e : f g h i j : k l m n o : p q r s t : ... > 6 disks: a b c d e f : g h i j k l : m n o p q r : s t : ... > > If it stopped at "t", both have "r s t"... and that "r s t" > would be what you have to find. You have to be wary of false > matches though (such as zeroes or other common patterns of data > you might find anywhere). > > And there's one more thing, you mentioned a disk had a bad sata cable. > If that disk got kicked and reshape went on for a while afterwards, > you should take that disk out of consideration. (Specify as "missing" > when creating the arrays.) It will have outdated/unreshaped data on > it that would be hard to incorporate into your recovery attempt... The bad SATA cable has been there for a while and reported spurious I/O errors on the old 5 disk raid. It really started to produce I/O errors when I placed the new drive in that slot. Is it OK to specify the new drive as missing? No special information that should be located on that drive? I wrote a small program to help me locate the correct offset: https://gist.github.com/Risca/3eda5e7aba3dc6b72d61e79eaf7cc147 I am on the right track with that program? > > Regards > Andreas Klauer > -- 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
