Dear people of linux-btrfs:

Thank you for btrfs! It is a beautiful thing. I say that in spite of
the fact that it seems to have failed and eaten some of my data.

I'm writing with two purposes: to get help and advice in recovering my
data, to help debug the software.

I was running linux 3.12.26 and btrfsprogs 3.14, and I started getting
error messages like these in my syslog:

syslog.7:Aug 16 02:32:35 spark kernel: [48524.140611] btrfs no csum
found for inode 15537898 start 4096

It happened only for one of the three partitions on this SSD, and
smartctl indicated no problem with the disk:

SMART overall-health self-assessment test result: PASSED
…
Num  Test_Description    Status                  Remaining
LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%      6406         -
# 2  Extended captive    Completed without error       00%      6405         -

I upgraded my kernel to 3.16.1 and tried the various techniques
suggested in https://btrfs.wiki.kernel.org/index.php/Btrfsck and
https://btrfs.wiki.kernel.org/index.php/Problem_FAQ , including
`btrfsck check --repair --init-csum-tree`. This didn't fix it.

I made an image of the filesystem in case someone wants to diagnose it
(78 MB), and I also a made a dd copy of the affected partition.

The `btrfs restore` command aborts even though I've passed the -i
flag. In fact, I see that on subsequent runs it aborts at different
places.

Looking at the source code
(http://git.kernel.org/cgit/linux/kernel/git/mason/btrfs-progs.git/tree/cmds-restore.c?id=c17d0a73c11d7cdbdf1582408ec6d168876160ea#n819)
I don't see how -6 from decompress could cause it to stop when I have
set `ignore_errors`, so next I ran it under valgrind.

Aha. When it is run under valgrind it consistently stops (killing
valgrind, in fact!) in the same way on every run.

Here's the tail of stdout and stderr when it aborted when run under valgrind:

Restoring ./sda6-btrfs-restore-3/@home/zooko/.mozilla/firefox/ltjwtkwe.ketotic.org/thumbnails/188888af64f6d2871b0f24e325d8a298.png
Restoring ./sda6-btrfs-restofailed to inflate: -6

Full valgrind outputs from such a run is attached to this letter.

I've spent a little time looking at the stack traces in the valgrind
log, and I *guess* that there is corruption such that the
decompression fails, and I guess it would be possible to make
cmds-restore handle corrupted compressedtext better, so that it would
end up skipping whatever files and directories were unrestorable due
to corruption. However, I don't immediately see how to proceed.

Regards,

Zooko Wilcox-O'Hearn

Founder, CEO, and Customer Support Rep
https://LeastAuthority.com
Freedom matters.