Currently, low disk space conditions only cause write operations to fail when the actual attempt to get/use more disk space from the OS fails. It would help (operationally) if writes could additionally fail when the amount of free disk space falls below some configurable threshold (eg. 10GB). This would leave some disk space as headroom that can be used to restore service, as opposed to the alternative of a filesystem that is either completely full or has a uselessly small amount of free space (eg. under 2GB).

A simple workaround for this (and the behaviour that I'm effectively asking for) is to allocate on disk a large dummy file of the desired size. This will cause the filesystem to fill up "early", and if this happens the dummy file can be deleted to gain access to the "spare" disk space.