12.08.2018 10:04, Andrei Borzenkov пишет:

On ZFS snapshots are contained in dataset and you limit total dataset
space consumption including all snapshots. Thus end effect is the same -
deleting data that is itself captured in snapshot does not make a single
byte available. ZFS allows you to additionally restrict active file
system size ("referenced" quota in ZFS) - this more closely matches your
expectation - deleting file in active file system decreases its
"referenced" size thus allowing user to write more data (as long as user
does not exceed total dataset quota). This is different from btrfs
"exculsive" and "shared". This should not be hard to implement in btrfs,
as "referenced" simply means all data in current subvolume, be it
exclusive or shared.

Oops, actually this is exactly what "referenced" quota is. Limiting
total subvolume + snapshots is more difficult, as there is no inherent
connection between qgroups of source and snapshot nor any built-in way
to include snapshot qgroup in some common total qgroup when creating
snapshot.