I think we do this, but making the threshold configurable would be good.

Boinc (a software unrelated to MC) use three settings to manage the disk usage in addition to the obvious "no space left":

• Use no more than value GB
• Leave at least value GB free
• Use no more than value % of total

All may be useful for hosted server.

Two config option have been added.
minimum_free_space: If a backup's creation would leave less than this amount of disk space remaining, the backup will be aborted
free_space_if_needed: If the previous backup failed due to lack of space, the oldest backup will be deleted to free space.

Fixed in 1.0.19