freebsd stable - Jan 2013 - Deleting the top-level ZFS file system (without affecting its children)

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

On Fri, 11 Jan 2013 16:11:32 +0100, xenophon\+freebsd  
<xenophon+freebsd at irtnog.org> wrote:
> 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
>
Why would rm -rf /oldroot/* not return all the allocated space?
I can only think of snapshots keeping the space allocated, but you can  
remove those too.
Can you elaborate on that?

Ronald.