parent_delta
delta = parent_delta(ds, used);
delta = parent_delta(ds, -used);
asize - MIN(asize, parent_delta(ds, asize + inflight));
uint64_t parent_rsrv = parent_delta(dd, used_on_disk + est_inflight,
parent_space = parent_delta(dd, est_used, space);
parent_delta(dd, dsl_dir_phys(dd)->dd_used_bytes, used);
delta = parent_delta(dd, dsl_dir_phys(dd)->dd_used_bytes, delta);