Kernel 4.8 and newer have a new ticketed enospc infrastructure that should avoid this, there's also earlier changes where empty block groups are deallocated aut

Kernel 4.8 and newer have a new ticketed enospc infrastructure that should avoid this, there's also earlier changes where empty block groups are deallocated automatically. If you're already using 4.8+ then try 4.12.6 or newer, which includes commit 4a309747. Due to past long standing enospc challenges, more experienced users are in the (bad) habit of micro-managing Btrfs manually with filtered balance, e.g. -dusage=15 -musage=15. Really we kinda need to stop doing that to find and fix the remaining edge cases; or worst case upstream should deploy a systemd timer / cron job to do these partial balances maybe once a week. Anyway I don't balance at all on my drives and haven't run into enospc in quite a while. So chances are you're hitting an edge case, and there's a bug that needs to be tracked down if you're already using 4.12.6+.