ZFS ashift?

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've ascertained that ZFS appears to be able to correctly determine native 
sector size. On a Xen DomU I get:
# zdb | grep ashift
            ashift: 9

On a physical machine with a disk with 4k native sectors I get:
# zdb | grep ashift
            ashift: 12

However, I cannot see a way to override this when creating a pool. The 
options seen in wider ZFS documentation are missing:
# zpool get all | grep ashift
#

# zpool create -o ashift=12 tank xbd1d
property 'ashift' is not a valid pool property
#

No vfs.zfs.min_auto_ashift sysctl:
# sysctl vfs.zfs
vfs.zfs.version.ioctl = 7
vfs.zfs.version.spa = 5000
#

--
Stephen