With a relatively recent versions of Solaris or OpenSolaris (say Nevada build 36, Solaris Express 4/06 and Solaris 10 6/06 release), here is how it is possible to use ZFS as the backing file system for such a removable device, easily.

The first step is certainly to disable the vold(1M) SMF service, for the operating system not trying to mount it automatically each time this device will be plugged in:

# svcadm disable volfs

Knowing that Solaris USB driver presents any USB storage device as removable media, it can be seen using both the format command in expert mode, and the rmformat program:

# format -e < /dev/null
Searching for disks...done

       0. c1d0 <DEFAULT cyl 9720 alt 2 hd 255 sec 63>
       1. c2d0 <ST325082-         4ND0XKT-0001-232.89GB>
       2. c3t0d0 <ST940211-5A-0000-37.26GB>
Specify disk (enter its number):
# rmformat
Looking for devices...
     1. Logical Node: /dev/rdsk/c0t0d0p0
        Physical Node: /pci@0,0/pci-ide@6/ide@0/sd@0,0
        Connected Device: LITE-ON  DVD SOHD-16P9S   F3S2
        Device Type: DVD Reader
        Bus: IDE
        Size: <Unknown>
        Label: <Unknown>
        Access permissions: <Unknown>
     2. Logical Node: /dev/rdsk/c3t0d0p0
        Physical Node: /pci@0,0/pci108e,5347@2,1/storage@3/disk@0,0
        Connected Device: ST940211 5A               0000
        Device Type: Removable
        Bus: USB
        Size: 38.2 GB
        Label: <Unknown>
        Access permissions: Medium is not write protected.

Now that the device name is clearly identified, it possible to create a specialized pool:

# zpool create rmzp c3t0d0
# zpool list rmzp
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
rmzp                     37G   28.6M   37.0G     0%  ONLINE     -

Since the purpose of this disk is, among other things, to be a backup of my home directory, here is how to do so.

First, create the correct zfs hierarchy on the USB disk:

# zfs create rmzp/home
# zfs set compression=on rmzp/home

Take a snapshot of current home directory, send and restore it on the fly to the new pool:

# zfs snapshot datazp/home/jgabel@rmzp.0
# zfs send datazp/home/jgabel@rmzp.0 | zfs receive rmzp/home/jgabel
# zfs destroy rmzp/home/jgabel@rmzp.0
# zfs list -r rmzp
rmzp                  28.6M  36.4G  25.5K  /rmzp
rmzp/home             28.5M  36.4G  26.5K  /rmzp/home
rmzp/home/jgabel      28.5M  36.4G  28.5M  /rmzp/home/jgabel

Then, when you are ready to take it away, just export the pool as with a classical disk:

# zpool export rmzp
# zpool list rmzp
cannot open 'rmzp': no such pool
# zpool import
  pool: rmzp
    id: 1670601809438763813
 state: ONLINE
action: The pool can be imported using its name or numeric identifier.

        rmzp        ONLINE
          c3t0d0    ONLINE

Wow! What an easy and powerful way to do backups on removable media, isn't it?

Since there is new ZFS porting work on the way (e.g. ZFS on FUSE/Linux and ZFS on FreeBSD), we can expect to share this kind of devices between Unix-like OSes very quickly.