Rescue a single-device btrfs instance with zeroed tree root

Rescue a single-device btrfs instance with zeroed tree root

[Ivan Shapovalov]

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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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Re: Rescue a single-device btrfs instance with zeroed tree root

[Ivan Shapovalov]

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On 2016-06-21 at 20:23 +0300, Ivan Shapovalov wrote:
> 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.


Anyone? I'd appreciate any advice on how to rebuild the tree roots.

I'd even write some code if someone tells me the disk format and
logical tree constraints (i. e. in which order to put pointers to child
nodes).

BTW, it looks like all tree roots are lost, i. e. btrfs-find-root with
any objectid (extent tree, subvolume tree, anything in ctree.h) finds
only level 0 nodes.

It should be possible to rebuild intermediate nodes, isn't it? Do they
contain valuable information?

Please Cc: me in replies as I'm not subscribed to the list.

Thanks,
-- 

Ivan Shapovalov / intelfx /

> 
> 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,

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