On Sun, Jun 12, 2016 at 11:22 PM, Maximilian Böhm <winlux@xxxxxxxxx> wrote:
> Hi there, I did something terribly wrong, all blame on me. I wanted to
> write to an USB stick but /dev/sdc wasn't the stick in this case but
> an attached HDD with GPT and an 8 TB btrfs partition…
GPT has a secondary copy at the end of the device, so maybe gdisk can
reconstruct in first main one at the beginning of the disk, I don't
know all gdisk commands. But is you once created just 1 partition max
size for the whole disk with modern tools, your btrfs fs starts at
sector 2048, (with logical sectorsize 512).
> $ sudo dd bs=4M if=manjaro-kde-16.06.1-x86_64.iso of=/dev/sdc
> 483+1 Datensätze ein
> 483+1 Datensätze aus
> 2028060672 bytes (2,0 GB, 1,9 GiB) copied, 16,89 s, 120 MB/s
To confuse btrfs+tools as little as possible, I would first overwrite
/dev/sdc again from the start with the same amount of bytes byte then
from /dev/zero.
Then create / 'newly overlay' a the original partition offset 1M, till
the end. Alternatively:
$ losetup /dev/loopX -o 1M /dev/sdc
then your broken fs will be on /dev/sdc1 or /dev/loopX
Or overlay it with dm, snapshot, set the original ro and then work on
the rw flavor, so you keep the current broken HDD/fs as is and then do
the above.
> $ sudo btrfs check --repair /dev/sdc
> enabling repair mode
> No valid Btrfs found on /dev/sdc
> Couldn't open file system
Forget --repair I would say, hopefully btrfs restore can still find /
copy most of the data
> $ sudo btrfs-find-root /dev/sdc
> No valid Btrfs found on /dev/sdc
> ERROR: open ctree failed
>
> $ sudo btrfs-show-super /dev/sdc --all
> superblock: bytenr=65536, device=/dev/sdc
> ---------------------------------------------------------
> ERROR: bad magic on superblock on /dev/sdc at 65536
>
> superblock: bytenr=67108864, device=/dev/sdc
> ---------------------------------------------------------
> ERROR: bad magic on superblock on /dev/sdc at 67108864
>
> superblock: bytenr=274877906944, device=/dev/sdc
> ---------------------------------------------------------
> ERROR: bad magic on superblock on /dev/sdc at 274877906944
run
$ sudo btrfs-show-super /dev/sdc1 --all
or
$ sudo btrfs-show-super /dev/loopX --all
> System infos:
>
> $ uname -a
> Linux Mongo 4.6.2-1-MANJARO #1 SMP PREEMPT Wed Jun 8 11:00:08 UTC 2016
> x86_64 GNU/Linux
>
> $ btrfs --version
> btrfs-progs v4.5.3
>
> Don't think dmesg is necessary here.
>
>
> OK, the btrfs wiki says there is a second superblock at 64 MiB
> (overwritten too in my case) and a third at 256 GiB ("0x4000000000").
> But how to restore it? And how to restore the general btrfs header
> metadata? How to restore GPT without doing something terrible again?
See above, maybe you have lookup various sizes etc GPT etc first, but
I think your 3rd SB should be there.
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