Sorry for my disturbance!

my disk with btrfs filesystem tell me it doesn't have enough free space:

```
$ sudo btrfs fi df /
Data, single: total=448.02GiB, used=427.74GiB
System, DUP: total=8.00MiB, used=64.00KiB
Metadata, DUP: total=8.80GiB, used=6.39GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
$ sudo btrfs fi usage /
Overall:
    Device size:                 465.64GiB
    Device allocated:            465.63GiB
    Device unallocated:            1.00MiB
    Device missing:                  0.00B
    Used:                        440.54GiB
    Free (estimated):             20.26GiB      (min: 20.26GiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)
    Multiple profiles:                  no

Data,single: Size:448.02GiB, Used:427.76GiB (95.48%)
   /dev/sda2     448.02GiB

Metadata,DUP: Size:8.80GiB, Used:6.39GiB (72.60%)
   /dev/sda2      17.60GiB

System,DUP: Size:8.00MiB, Used:64.00KiB (0.78%)
   /dev/sda2      16.00MiB

Unallocated:
   /dev/sda2       1.00MiB
$ sudo btrfs fi show /
Label: none  uuid: 644a9e91-5e05-4f5d-a79b-e0eccd21a1a8
        Total devices 1 FS bytes used 434.14GiB
        devid    1 size 465.64GiB used 465.63GiB path /dev/sda2

$ df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
devtmpfs       devtmpfs  3.8G     0  3.8G   0% /dev
tmpfs          tmpfs     3.9G   46M  3.8G   2% /dev/shm
tmpfs          tmpfs     3.9G  3.9M  3.9G   1% /run
/dev/sda2      btrfs     466G  442G   21G  96% /
tmpfs          tmpfs     3.9G  204K  3.9G   1% /tmp
/dev/sda1      vfat      128M  125M  2.9M  98% /boot
tmpfs          tmpfs     785M   16K  785M   1% /run/user/994
tmpfs          tmpfs     785M   80K  785M   1% /run/user/1000
```

however, i try to use gdu to scan my disk. it tell me

```
$ sudo gdu /
 gdu ~ Use arrow keys to navigate, press ? for help
 --- / ---
   49.9 GiB [#####     ] /usr
   15.1 GiB [#         ] /home
   11.1 GiB [#         ] /opt
    6.5 GiB [          ] /var
  930.8 MiB [          ] /lib
  124.9 MiB [          ] /boot
   55.2 MiB [          ] /root
   50.6 MiB [          ] /etc
   27.1 MiB [          ] /sbin
   23.5 MiB [          ] /lib64
   10.9 MiB [          ] /bin
. 312.0 KiB [          ] /tmp
   20.0 KiB [          ] /mnt
e   4.0 KiB [          ] /srv
e   4.0 KiB [          ] /tftproot
@   4.0 KiB [          ] media
 Total disk usage: 83.8 GiB Apparent size: 78.4 GiB Items: 2424046
Sorting by: size desc
```

427.76GiB or 83.8 GiB!

i know btrfs is a CoW filesystem, which means it will cost more space
to store a file. but the gap is too large, which it is hard to accept
the fact.
who can tell me what happened to my disk? and how can i do to rescue
my hard disk? the rest free space is not enough. if i cannot find the
solution, maybe i should format my hard disk to get more free space.
thanks.

there are some information which maybe useful.

```
$ uname -r
5.11.7-gentoo-x86_64
$ btrfs --version
btrfs-progs v5.9
$ dmesg > dmesg.txt && wgetpaste -s dpaste dmesg.txt
Pasting > 25 kB often tend to fail with dpaste. Use --verbose or
--debug to see the
error output from wget if it fails. Alternatively use another pastebin service.
Your paste can be seen here: https://dpaste.com/FCZF83YYA
```