Ceph » CephFS

Currently dentries are hashed among dirfrags using the linux dcache's hash function, which is pretty trivial. The problem is that it very poorly distributes regularly named files (things like file.%d).

Add a field to the dir's layout that selects an alternate hash function. This will be somewhat backward incompatible/non-optimal, because old clients will sent requests to the wrong mds, and then they'll be forwarded. When teh directory is fragmented and has a different hash selected, at least.

See mds_dir_hash branch.