zfs dataset seems empty (while it is not) after reboot?

Hello,

After I copied data to /data and portsnap fetch extract filled 
/usr/ports I get:


# zfs list /usr/ports
NAME                 USED  AVAIL     REFER  MOUNTPOINT
zroot/ROOT/default  1.49G   202G     1.49G  /

# ls -l /usr/ports
total 0

# zfs list /data
NAME                 USED  AVAIL     REFER  MOUNTPOINT
zroot/ROOT/default  1.49G   202G     1.49G  /

ls -l /data
total 0



zfs list shows data in plce but ls shows nothing.

I'm not sure what I'm doing wrong ?