spa_get_slop_space
resv = spa_get_slop_space(spa);
if (free_space <= spa_get_slop_space(spa) || vd->vdev_removing) {
extern uint64_t spa_get_slop_space(spa_t *spa);
if (available < vd->vdev_stat.vs_dspace + spa_get_slop_space(spa)) {