btrfs check --repair: ERROR: cannot read chunk root

[Marc MERLIN <marc@merlins.org>]

I have a filesystem on top of md raid5 that got a few problems due to the
underlying block layer (bad data cable).
The filesystem mounts fine, but had a few issues
Scrub runs (I didn't let it finish, it takes a _long_ time)
But check --repair won't even run at all:

myth:~# btrfs --version
btrfs-progs v4.7.3
myth:~# uname -r
4.8.5-ia32-20161028

myth:~# btrfs check -p --repair  /dev/mapper/crypt_bcache0  2>&1 | tee
/var/spool/repair
bytenr mismatch, want=13835462344704, have=0
ERROR: cannot read chunk root
Couldn't open file system
enabling repair mode
myth:~#

myth:~# btrfs rescue super-recover -v /dev//mapper/crypt_bcache0 
All Devices:
        Device: id = 1, name = /dev//mapper/crypt_bcache0

Before Recovering:
        [All good supers]:
                device name = /dev//mapper/crypt_bcache0
                superblock bytenr = 65536

                device name = /dev//mapper/crypt_bcache0
                superblock bytenr = 67108864

                device name = /dev//mapper/crypt_bcache0
                superblock bytenr = 274877906944

        [All bad supers]:

All supers are valid, no need to recover


I don't care about the data, it's a backup array, but I'd still like to know
if I can recover from this state and do a repair to see how much data got
damaged

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/