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 w

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.