Dell PowerVault 720N Command Reference Manual page 255

Before any snapshot
is taken, disk space
is consumed by the
active file system
only.
Space used by the active file system
Space used by the snapshot only
Unused disk space
After a snapshot is
taken, the active file
system and snapshot
point to the same disk
blocks. The snapshot
doesn´t use extra disk
space.
foo
foo
After the foo file is
deleted, the space
previously used by foo
is referenced by the
snapshot. That´s why
deleting active file
system data doesn´t
free disk space.
foo
foo