BTRFS unknown space usage

Question

I have 1 TB hard drive with btrfs on it used for /home. When moving to new ubuntu (with fresh install), I've moved my @home subvolume to @home-old, installed and configured system, copied needed files onto new @home and removed @home-old with btrfs subvol delete --commit.

Now I have high space usage for seemengly no reason. That's what btrfs commands say (I did some balancing, root subvolume is mounted at /mnt):

root@v-home:~# sudo btrfs filesystem usage -h -T /mnt
Overall:
    Device size:         931.51GiB
    Device allocated:        751.07GiB
    Device unallocated:      180.44GiB
    Device missing:          0.00B
    Used:            744.63GiB
    Free (estimated):        184.82GiB  (min: 94.60GiB)
    Data ratio:               1.00
    Metadata ratio:           2.00
    Global reserve:      512.00MiB  (used: 0.00B)

            Data      Metadata System               
Id Path     single    DUP      DUP       Unallocated
-- -------- --------- -------- --------- -----------
 1 /dev/sdb 741.01GiB 10.00GiB  64.00MiB   180.44GiB
-- -------- --------- -------- --------- -----------
   Total    741.01GiB  5.00GiB  32.00MiB   180.44GiB
   Used     736.63GiB  4.00GiB 112.00KiB            
root@v-home:~# btrfs filesystem du -s /mnt/@
@backups/ @home/    
root@v-home:~# btrfs filesystem du -s /mnt/*
     Total   Exclusive  Set shared  Filename
     0.00B       0.00B       0.00B  /mnt/@backups
 412.11GiB   149.73GiB   197.05GiB  /mnt/@home

Why data usage is much bigger than Total for /mnt/@home? Is there a way to fix this?

Answer 1

Even with the --commit flag, btrfs does not immediately deallocate the used space, because it's a very slow operation. After deleting a large subvolume, you will notice a btrfs-cleaner task in the process list which is responsible for this operation.

To wait until the subvolume is actually completely deleted, use btrfs subvolume sync.