Comment # 2 on bug 1009339 from Achim Gratz

I've created an image of the root while it was read-only and tried to repair
that:

# btrfs rescue zero-log sysroot.img
Clearing log on sysroot.img, previous log_root 0, level 0
# btrfs rescue chunk-recover sysroot.img
Scanning: DONE in dev0                       
We are going to rebuild the chunk tree on disk, it might destroy the old
metadata on the disk, Are you sure? [y/N]: y
Chunk tree recovered successfully
Gertrud:~ # btrfs check sysroot.img
Checking filesystem on sysroot.img
UUID: 95cadcea-1e91-4624-aab9-859ea02ad06f
Chunk[256, 228, 543533039616] stripe[1, 60167290880] is not found in dev extent
Chunk[256, 228, 562659065856] stripe[1, 81642127360] is not found in dev extent
Chunk[256, 228, 568094883840] stripe[1, 62314774528] is not found in dev extent
Chunk[256, 228, 569336397824] stripe[1, 67683483648] is not found in dev extent
Chunk[256, 228, 575509364736] stripe[1, 42987421696] is not found in dev extent
Chunk[256, 228, 577656848384] stripe[1, 58019807232] is not found in dev extent
Chunk[256, 228, 585844129792] stripe[1, 69830967296] is not found in dev extent
Chunk[256, 228, 615316979712] stripe[1, 6614417408] is not found in dev extent
ref mismatch on [638004637696 143360] extent item 2, found 3
Backref 638004637696 root 282 owner 30677 offset 0 num_refs 0 not found in
extent tree
Incorrect local backref count on 638004637696 root 282 owner 30677 offset 0
found 1 wanted 0 back 0x19ccd40
Backref bytes do not match extent backref, bytenr=638004637696, ref
bytes=143360, backref bytes=880640
backpointer mismatch on [638004637696 143360]
ref mismatch on [638048546816 4096] extent item 0, found 1
Backref 638048546816 root 282 owner 30689 offset 0 num_refs 0 not found in
extent tree
Incorrect local backref count on 638048546816 root 282 owner 30689 offset 0
found 1 wanted 0 back 0xba78b10
backpointer mismatch on [638048546816 4096]
checking free space cache
checking extentscache and super generation don't match, space cache will be
invalidated
checking fs roots
checking csums
checking root refs
checking quota groups
ERROR: out of memory
ERROR: Loading qgroups from disk: -2
found 34938126339 bytes used err is 0
total csum bytes: 28927260
total tree bytes: 1704460288
total fs tree bytes: 1593638912
total extent tree bytes: 68534272
btree space waste bytes: 246374393
file data blocks allocated: 846205018112
 referenced 95776739328
extent buffer leak: start 141427359744 len 16384
extent buffer leak: start 141426868224 len 16384
btrfs check --repair sysroot.img
enabling repair mode
Checking filesystem on sysroot.img
UUID: 95cadcea-1e91-4624-aab9-859ea02ad06f
Chunk[256, 228, 543533039616] stripe[1, 60167290880] is not found in dev extent
Chunk[256, 228, 562659065856] stripe[1, 81642127360] is not found in dev extent
Chunk[256, 228, 568094883840] stripe[1, 62314774528] is not found in dev extent
Chunk[256, 228, 569336397824] stripe[1, 67683483648] is not found in dev extent
Chunk[256, 228, 575509364736] stripe[1, 42987421696] is not found in dev extent
Chunk[256, 228, 577656848384] stripe[1, 58019807232] is not found in dev extent
Chunk[256, 228, 585844129792] stripe[1, 69830967296] is not found in dev extent
Chunk[256, 228, 615316979712] stripe[1, 6614417408] is not found in dev extent
ref mismatch on [638004637696 143360] extent item 2, found 3
attempting to repair backref discrepency for bytenr 638004637696
Backrefs don't agree with each other and extent record doesn't agree with
anybody, so we can't fix bytenr 638004637696 bytes 143360
failed to repair damaged filesystem, aborting
checking extents

It seems the error is somewhere in the snapshots subvolume (282).  I'm started
from snapshot 1974 since I had to rollback a botched update some time ago, so I
need to keep that or somehow create a new sysroot image that just contains the
currently active files in order to replace the failed btrfs on the sysroot
volume.