Rescue a single-device btrfs instance with zeroed tree root

[Ivan Shapovalov <intelfx@intelfx.name>]

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Hello,

So this is another case of "I lost my partition and do not have
backups". More precisely, _this_ is the backup and it turned out to be
damaged.

(The backup was made by partclone.btrfs. Together with a zeroed out
tree root, this asks for a bug in partclone...)

So: the tree root is zeroes, backup roots are zeroes too,
btrfs-find-root only reports blocks of level 0 (needed is 1).
Is there something that can be done? Maybe it is possible to
reconstruct the root from its children?
Operations log following.
Please Cc: me in replies as I'm not subscribed to the list.

1. regular mount

# mount /dev/loop0p3 /mnt/temp

=== dmesg ===
[106737.299592] BTRFS info (device loop0p3): disk space caching is enabled
[106737.299604] BTRFS: has skinny extents
[106737.299884] BTRFS error (device loop0p3): bad tree block start 0 162633449472
[106737.299888] BTRFS: failed to read tree root on loop0p3
[106737.314359] BTRFS: open_ctree failed
=== end dmesg ===

2. mount with -o recovery

# mount -o recovery /dev/loop0p3 /mnt/temp

=== dmesg ===
[106742.305720] BTRFS warning (device loop0p3): 'recovery' is deprecated, use 'usebackuproot' instead
[106742.305722] BTRFS info (device loop0p3): trying to use backup root at mount time
[106742.305724] BTRFS info (device loop0p3): disk space caching is enabled
[106742.305725] BTRFS: has skinny extents
[106742.306056] BTRFS error (device loop0p3): bad tree block start 0 162633449472
[106742.306060] BTRFS: failed to read tree root on loop0p3
[106742.306069] BTRFS error (device loop0p3): bad tree block start 0 162633449472
[106742.306071] BTRFS: failed to read tree root on loop0p3
[106742.306084] BTRFS error (device loop0p3): bad tree block start 0 162632237056
[106742.306086] BTRFS: failed to read tree root on loop0p3
[106742.306097] BTRFS error (device loop0p3): bad tree block start 0 162626682880
[106742.306100] BTRFS: failed to read tree root on loop0p3
[106742.306111] BTRFS error (device loop0p3): bad tree block start 0 162609168384
[106742.306114] BTRFS: failed to read tree root on loop0p3
[106742.327272] BTRFS: open_ctree failed
=== end dmesg ===


3. btrfs-find-root

# btrfs-find-root /dev/loop0p3
Couldn't read tree root
Superblock thinks the generation is 22332
Superblock thinks the level is 1
Well block 162633646080(gen: 22332 level: 0) seems good, but generation/level doesn't match, want gen: 22332 level: 1
Well block 162633596928(gen: 22332 level: 0) seems good, but generation/level doesn't match, want gen: 22332 level: 1
Well block 162633515008(gen: 22332 level: 0) seems good, but generation/level doesn't match, want gen: 22332 level: 1

Thanks,
-- 

Ivan Shapovalov / intelfx /

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