btrfs suddenly lost all om my huge free space

[Tommy Pettersson <ptp@lysator.liu.se>]

Hi,

(I'm not subscribed to the list, so please CC me.)

I have a btrfs with raid1 on two identical unpartitioned disks.
Today I noticed that df (normal df) said I am 77 % full. This
was a chock, because since forever it has been around 12 %.


# btrfs fi show
Label: 'green'  uuid: dd83031c-2447-4736-a8f6-9bd9cdeea879
        Total devices 2 FS bytes used 212.88GB
        devid    2 size 1.82TB used 356.04GB path /dev/sdb
        devid    1 size 1.82TB used 356.06GB path /dev/sda

# btrfs fi df /
Data, RAID1: total=276.00GB, used=209.02GB
Data: total=8.00M, used=0.00
System, RAID1: total=40.00MB, used=64.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=80.00GB, used=3.88GB
Metadata: total=8.00MB, used=0.00

# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          3.7T  426G   134G  77% /


The thing that has drastically changed is Avail in the output
from df.

I tried a btrfs balance, which self-aborted after some hours
with No space left on device. I deleted two snapshots, so I got
some free space and could use the system again.

The balance, although it didn't finish, seems to have reduced
the used space, but it also reduced the "available" space:


# btrfs fi show
Label: 'green'  uuid: dd83031c-2447-4736-a8f6-9bd9cdeea879
        Total devices 2 FS bytes used 212.88GB
        devid    2 size 1.82TB used 356.04GB path /dev/sdb
        devid    1 size 1.82TB used 215.01GB path /dev/sda

# btrfs fi df /
Data, RAID1: total=210.00GB, used=197.97GB
System, RAID1: total=8.00MB, used=44.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=5.00GB, used=3.41GB

# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          3.7T  403G   25G  95% /


I made an unqualified guess that the space cache was corrupted,
and tried to mount with option clear_cache and nospace_cache.
Both of them caused btrfs to scan my disks for a couple of
minutes at boot, but the amount of available space did not
improve.

What can I do to help locate the cause of this problem?


Regards,
Tommy