hammer pfs-destroy failed: Directory not empty

[lhmwzy]

 uname -a:
DragonFly reli.com 3.6-RELEASE DragonFly v3.6.1.7.ga64375-RELEASE #0: Thu
Mar 13 10:56:17 CST 2014     lhm at reli.com:/usr/obj/usr/src/sys/lhmwzy
x86_64


# mount
ROOT on / (hammer, local)
devfs on /dev (devfs, local)
/dev/da0s1a on /boot (ufs, local)
/pfs/@@-1:00001 on /var (null, local)
/pfs/@@-1:00002 on /tmp (null, local)
/pfs/@@-1:00003 on /usr (null, local)
/pfs/@@-1:00008 on /home (null, local)
/pfs/@@-1:00005 on /usr/obj (null, local)
/pfs/@@-1:00006 on /var/crash (null, local)
/pfs/@@-1:00007 on /var/tmp (null, local)
procfs on /proc (procfs, local)

# hammer info
Volume identification
        Label               ROOT
        No. Volumes         1
        FSID                0155c2ba-a8f9-11e3-90ec-010c29f0c123
        HAMMER Version      6
Big block information
        Total           14902
        Used              438 (2.94%)
        Reserved           23 (0.15%)
        Free            14441 (96.91%)
Space information
        No. Inodes     212220
        Total size       116G (125007036416 bytes)
        Used             3.4G (2.94%)
        Reserved         184M (0.15%)
        Free             113G (96.91%)
PFS information
        PFS ID  Mode    Snaps  Mounted on
             0  MASTER      0  /
             1  MASTER      7  /var
             2  MASTER      0  /tmp
             3  MASTER      7  /usr
             4  SLAVE       -  not mounted
             5  MASTER      0  /usr/obj
             6  MASTER     17  /var/crash
             7  MASTER      0  /var/tmp
             8  MASTER     15  /home

 ll /pfs
total 0
lrwxr-xr-x  1 root  wheel  10 Mar 20 10:08 home -> @@-1:00008
lrwxr-xr-x  1 root  wheel  10 Mar 11 16:42 tmp -> @@-1:00002
lrwxr-xr-x  1 root  wheel  10 Mar 11 16:42 usr -> @@-1:00003
lrwxr-xr-x  1 root  wheel  10 Mar 11 16:42 usr.obj -> @@-1:00005
lrwxr-xr-x  1 root  wheel  10 Mar 11 16:42 var -> @@-1:00001
lrwxr-xr-x  1 root  wheel  10 Mar 11 16:42 var.crash -> @@-1:00006
lrwxr-xr-x  1 root  wheel  10 Mar 11 16:42 var.tmp -> @@-1:00007

but
 # hammer pfs-destroy /pfs/@@-1:00004
You have requested that PFS#4 () be destroyed
This will irrevocably destroy all data on this PFS!!!!!
Do you really want to do this? y
Destroying PFS #4 () in  5 4 3 2 1.. starting destruction pass
pfs-destroy of PFS#4 failed: Directory not empty


Why and how to destroy PFS #4?
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