Dirhash

Dirhash is a feature of FreeBSD that improves the speed of finding files in a directory. Rather than finding a file in a directory using a linear search algorithm, FreeBSD uses a hash table^[1]. The feature is backwards-compatible because the hash table is built in memory when the directory is accessed, and it does not affect the on-disk format of the filesystem.

References

1. ^ Recent Filesystem Optimisations in FreeBSD, by Ian Dowse and David Malone