If a pool has no overprovision set, then it has a real hard limit on the amount of space left to place filesystems, how to show to users?

[mulkieran]

It will be necessary to put the specific value on the D-Bus, rather than constituent values. The calculation is, essentially, space actually available in the backstore (-, e.g., Stratis metadata) - (Stratis overhead, i.e. mdv and thin metas). This leaves a region that, at this time, is simply the space to which the thinpool could be extended, assuming there is no additional space required for the thin meta.