Deleting the top-level ZFS file system (without affecting its children)

[xenophon\+freebsd]

When I originally set up ZFS on my server, I used the topmost file
system for the root file system.  Last night, I used "zfs send" and "zfs
recv" to create a new root file system named "zroot/root".  Then, I
adjusted the mount points in single-user mode.  Based on my reading of
the contents of src/sys/boot/zfs/ and src/sys/boot/i386/zfsboot/
(specifically the zfs_mount() and zfs_get_root() functions in
zfsimpl.c), I ran "zpool set bootfs=zroot/root zroot".  This should
allow the boot program to find the new root file system.

Now, I'd like to delete the old root file system and return its storage
to the pool.  Clearly, "rm -rf /oldroot/*" wouldn't return the space
already allocated to the old root file system, but I don't want to run
"zfs destroy zroot", as that will probably affect its children (the
whole rest of the pool).  At this point, I suspect that I'd have to
re-create the pool to get the desired configuration.

Is my understanding correct?

Right now, the pool's datasets look something like the following:

  xenophon at cinep001bsdgw:~>zfs list
  NAME           USED  AVAIL  REFER  MOUNTPOINT
  zroot         75.5G   143G  1.04G  /oldroot
  zroot/root    1.04G   143G  1.03G  /
  zroot/usr     28.6G   143G  10.2G  /usr
  (etc.)

Best wishes,
Matthew

-- 
I FIGHT FOR THE USERS