Lvm – Recovering a mdadm+lvm+ext4 partition with read error

lvmmdadmsynology

One of disks in my NAS has failed.
The NAS is running Linux, and it uses mdadm + LVM technology for its filesystems.

I do have backup for most of the contents, but not for the very last changes, and if possible, I'd like to recover that from this failing disk.

The disk (a 'green drive' WD10EARS 1TB in size) throws this kind of errors:

Oct  3 12:00:41 kernel: [ 3625.620000] ata5.00: read unc at 9453282
Oct  3 12:00:41 kernel: [ 3625.620000] lba 9453282 start 9453280 end 1953511007 
Oct  3 12:00:41 kernel: [ 3625.620000] sde5 auto_remap 0
Oct  3 12:00:41 kernel: [ 3625.630000] ata5.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x6
Oct  3 12:00:41 kernel: [ 3625.630000] ata5.00: edma_err_cause=00000084 pp_flags=00000003, dev error, EDMA self-disable
Oct  3 12:00:41 kernel: [ 3625.640000] ata5.00: failed command: READ FPDMA QUEUED
Oct  3 12:00:41 kernel: [ 3625.650000] ata5.00: cmd 60/40:00:e0:3e:90/00:00:00:00:00/40 tag 0 ncq 32768 in
Oct  3 12:00:41 kernel: [ 3625.650000]          res 41/40:00:e2:3e:90/12:00:00:00:00/40 Emask 0x409 (media error) <F>
Oct  3 12:00:41 kernel: [ 3625.660000] ata5.00: status: { DRDY ERR }

However, while testing with 'dd', I noticed that if I skip the first 4kB, the read seems to be ok,
i.e. a command like.
dd if=/dev/sde5 of=dev/null bs=4k count=1000 skip=1
doesn't return any read error.

Supposing that there is no other read failure in the rest of the disk, would I be able to recover this 900 GB partition (as I mentioned before, it's a 'linux raid autodetect' partition, that contains a a LVM2 volume that contains a ext4 filesystem) if I copy-clone the partition somewhere else, but the first 4kB?

Best Answer

...Otherwise (i.e. if you already have degraded array) you still should be able to do it.

If your mdadm which created the array is < v.3.0, than the payload starts in offset 0x22000 (and further up if it is v.3.0 or later), which as well before the deleted first 4KB.

So only the linux-raid superblock got corrupted, but that is not that difficult to restore (especially if you have other devices in that array intact).

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