Prior to UFS2, the performance of name lookups in directories with many entries on FreeBSD could be quite slow. FFS does not maintain an on-disk index of directory entries, so each name lookup required a linear search of the directory entries. With UFS2 a new technique called dirhash was developed to generate a hash map in memory of directory entries the first time a name lookup is run on a large directory, so that subsequent name lookups will be much faster. Dirhash has had a substantial positive impact on filesystem performance.

However, the amount of memory which can be used by dirhashes is fixed by the kernel (although it is tunable at runtime via a sysctl). I propose to implement dynamic memory allocation for dirhashes, allowing more memory to be allocated for dirhash when needed if it is available, and reclaimed if memory becomes scarce. The goal of this is to improve performance for filesystems with very large directories on systems with memory to spare, without the user having to manually tune the amount of memory available for dirhash.

Furthermore, if time permits I would like to investigate implementing on-disk indexing for UFS2. If directory indices are generated and stored on disk, there would no longer be a need to generate a completely new dirhash each time a directory is first accessed after system startup, or after a dirhash for a directory not recently accessed has been disposed of to conserve memory.