[chungy]

ZFS reserves 1/64 of every disk precisely so it can't be truly fully allocated. It leaves enough room to delete snapshots, truncate files, and so forth.

Mind that everything is copy-on-write, you can't do anything, even metadata changes, without allocating new blocks. It needs the reserve space.

[tobias3]

I had a ZFS bug once where they increased the amount reserved in a new release which caused my file system to be 100% and me unable to delete anything until I went back to the previous release.

Btrfs uses the the disk completely. This is harder to do (also compared to e.g. ext4 reserving a fixed amount of inode space which may be unused when the disk is full). At some point they added an in-memory "global reserve" metadata space which allows you to delete stuff even if the file system is full.