Btrfs - why is the space not freed up?

Question

btrfs

There is btrfs on md raid. Free 0 bytes
/dev/md2 70T 70T 0 100% /volume1
I delete 5 gigabytes of files - the size does not change. Ok, I deleted the snapshot - the size does not change.
What is wrong with the file system and what can be done?

# btrfs fi show
Label: '2017.11.03-14:43:33 v15152'  uuid: b69bc514-443b-4da1-ab90-10ad664ac0e9
  Total devices 1 FS bytes used 69.91TiB
  devid    1 size 72.73TiB used 72.63TiB path /dev/md2

# btrfs fi df /volume1/
Data, single: total=69.84TiB, used=69.83TiB
System, DUP: total=8.00MiB, used=1.56MiB
Metadata, DUP: total=1.40TiB, used=84.80GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

PS The case is happening on synology, I tried to make btrfs balance on the advice from the Internet, in response I get a balance unexpected token.
It is also interesting that if you write data to disk via dd, then the file is created without errors. But the free space, as it was equal to 0, remains the same.

Answer 1

For those who encounter a similar problem in synology, there is no point in deleting snapshots through the terminal. Perform all operations through the snapshot replication application, after which the space will be freed up.