extern int vdev_open(vdev_t *);
extern void vdev_open_children(vdev_t *);
-extern boolean_t vdev_uses_zvols(vdev_t *);
extern int vdev_validate(vdev_t *, boolean_t);
extern void vdev_close(vdev_t *);
extern int vdev_create(vdev_t *, uint64_t txg, boolean_t isreplace);