Not that it counters any point you're making, but ZFS displays the number of contained entries of a directory in the directory's size field; mind that . and ..

Not that it counters any point you're making, but ZFS displays the number of contained entries of a directory in the directory's size field; mind that . and .. are included, so you usually need to subtract 2 to get the count you actually want. I do find it useful sometimes to know the count without getting the listing; the former is a very inexpensive operation (since ZFS is keeping track of metadata like you suggested), the latter is expensive, potentially extremely with hundreds of thousands or more of entries. This is more-or-less unique to ZFS. Other file systems even on Linux and FreeBSD generally don't provide this behavior.