On Sun, Jun 12, 2016 at 11:22 PM, Maximilian Böhm [off-list ref] 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.