lost zfs pool after buildworld

I had created a zfs pool of two disks (da1 and da2).
 
# zpool create tank mirror /dev/da1 /dev/da2
then created the dir
# zfs create tank/samba
and the mount point
#zfs set mountpoint=/usr/local/samba tank/samba
 
it all worked well.
this was after zfs was put to head the same hours after.
Now i have cvsupped the system did a buildworld and my pool is gone.
zpool list says no pool defined.
 
I did the installworld from a remote location so zfs was active during installworld, and zfs module loaded.
 
Must i unload zfs next time?
I have recreated the pool now ( no important data) but is this normal or is it bad luck.
 
And is there a way to let zpool see the metadata (if any) on the disk.
 
regards,
Johan