How to use hammer volume-add and volume-del

[Michael Neumann]

2010/4/7 lhmwzy <lhmwzy at gmail.com>
# uname -a
DragonFly . 2.6-RELEASE DragonFly v2.6.1-RELEASE #1: Sun Apr  4
19:50:41 PDT 2010
root at test28.backplane.com:/usr/obj/usr/src-misc/sys/GENERIC  i386

# hammer volume-del /dev/da1 /usr
hammer volume-del ioctl: Invalid argument/usr is not a valid HAMMER filesystem! You must specify the path you use in mount, not a PFS.For example:
  mount_hammer /dev/ad1 /mnt  hammer volume-add /dev/ad2 /mnt 
when added a disk,can't be removed.

another question,the same disk(/dev/da1 for example) can be added for
many times using "hammer volume-add /dev/da1 /usr",is this normal?What does hammer info /usr show? I am sure it is not added, but maybe noerror is shown.
Regards,  Michael