Reserve a portion of the storage to ensure that ENOSPC is recoverable

[DemiMarie]

Instead of relying on users to stop writing before the volume gets full, BTRFS could throw ENOSPC or just block writes until balancing completes.

[gerion0]

Came here to suggest the same after a BTRFS recovery session with a Live-CD. I thought about some form of "emergency mode" that allows just a few operations with exactly that reserved space. Something like that:

$ btrfs emergency-mode start
$ btrfs balance start
You are in emergency-mode. Balancing only data since metadata balancing needs more metadata.
...
$ btrfs subvolume create /foo
You are in emergency mode. Operations that allocate data are not allowed.
$ btrfs subvolume delete /some/snapshot
... deleted, but not synced ...
$ btrfs fi sync /
$ btrfs emergency-mode stop

[DemiMarie]

I don’t think an emergency mode should be needed. Balancing should always work, and if it doesn’t, it’s a BTRFS bug.