Comment # 5 on bug 1099769 from Bruno Friedmann

With tumbleweed snapshot 2018116 kernel 4.19.1
Hardware is Laptop Dell Precision 7510 with Xeon CPU E3-1535M v5 uefi boot
64GB Ram 2400 DDR4
Primary storage is Toshiba NVMe 1024GB (gpt)
p1 : 164MB vfat16 (efi)
p2 : 943GB luks encrypted / btrfs
p3 : 2GB luks encrypted swap
Grub is used to decrypted the root btrfs partition

I discover this morning a btrfs-transacti running at 100% (started by the btrfs
balance timer during the night)

After a reboot, the system is now unsable with error due to timeout on mounting
the btrfs root filesystem.

booting a rescue system (same tw snapshot) and trying to mount the fs manually
result in same process btrfs-transacti eating 100% of cpu.
tonight Mount finally didn't success after waiting more than 45 minutes.

When the btrfs partition was last mounted (even with usebackuproot) there was
btrfs balance running on

Trying to run a btrfs balance cancel /mnt lead to several backtrace in the
kernel log (see attached dmesg captured if one day I can get them out of the
broken fs)

It seems that there's not enough place on the partition even if df was telling
163GB free over the 943GB total of the partition.
>From memory as it is not possible to mount or grab information from the defect
volume.

BTW ; What's written in the wiki https://en.opensuse.org/SDB:BTRFS 
btrfs scrub start /dev/mapper/cr_nvme0n1p2 doesn't work it state error not
mounted filesystem ? What's the problem as it should normally possible to use a
device (here the cryptsetup luksOpen /dev/nvmen0p2 cr_nvme0n1p2 unlocked
device)

It's the first time since one year the system is installed, that this happen.
The only things that have changed, is a lot of newer data in (up to 85% used)
and numerous file and directories cleanup yesterday afternoon.

What is the best possible recovery scenario ? and the best forensic approach to
debug this nasty behaviour. any tricks is welcomed.

In the meantime I will at least retry to mount it ro and use a btrfs dump.