btrfs reported used space doesn't correspond with space occupied by the files themselves

[Daniel Martinez <danielsmartinez@gmail.com>]

Hello,

I've recently converted my root 32GB ext4 partition to btrfs (using
btrfs-progs 5.2). After that was done, I made a snapshot and tried to
update the system. Unfortunately I didn't have enough free space to
fit the whole update on that small partition, so it failed. I then
realized my mistake and deleted not only that newly made snapshot, but
also ext2_saved and some random files on the filesystem, totaling
about 5GB. For my surprise, the update still failed due to ENOSPC.

At this point, I tried running a balance, but it also failed with
ENOSPC. I tried the balance -dusage X with X increasing from zero, but
to my surprise again, it also failed.

Data, single: total=28.54GiB, used=28.34GiB
System, single: total=32.00MiB, used=16.00KiB
Metadata, single: total=1.00GiB, used=807.45MiB
GlobalReserve, single: total=41.44MiB, used=0.00B

Looking at btrfs filesystem df, it looks like those 5GB of data I
deleted are still occupying space. In fact, ncdu claims all the files
on that drive sum up to only 19GB.

I tried adding a second 2GB drive but that still wasn't enough to run
a full data balance (metadata runs fine).

This is what filesystem usage looks like:

Overall:
    Device size:                  31.59GiB
    Device allocated:             29.57GiB
    Device unallocated:            2.03GiB
    Device missing:                  0.00B
    Used:                         29.13GiB
    Free (estimated):              2.22GiB      (min: 2.22GiB)
    Data ratio:                       1.00
    Metadata ratio:                   1.00
    Global reserve:               41.44MiB      (used: 0.00B)

Data,single: Size:28.54GiB, Used:28.34GiB
   /dev/sda7     768.00MiB
   /dev/sdb1      27.79GiB

Metadata,single: Size:1.00GiB, Used:807.45MiB
   /dev/sdb1       1.00GiB

System,single: Size:32.00MiB, Used:16.00KiB
   /dev/sdb1      32.00MiB

Unallocated:
   /dev/sda7       1.03GiB
   /dev/sdb1       1.00GiB


I then made a read-only snapshot of the root filesystem and used btrfs
send/receive to transfer it to another btrfs filesystem, and when it
got there its also only occupying 19GB.

So there seems 10GB got lost somewhere in the process and I can't find
a way to get them back (other thank mkfs'ing and restoring a backup),
which in this case is about 30% of the available disk space.

What may be causing this?

Thanks,
Daniel.