PersistentStoreDir subdirs are not pruned after session has
expired?

I have a test clustering instance setup using PersistentStoreType of file.

I noticed that content of the PersistentStoreDir has multiple sub-dirs deep.
When the sesison is expired

  . The leaf file is deleted correctly

  . But the sub-dirs are not pruned.

After running for a while, I am seeing a large number sub directory under
PersistentStoreDir. Anyone knows if this is the expected behavior? Or if there
is a setting to allow all the sub-directory pruning.

I've search the newsgroup and doc but did not see any mention of this.