Postfix hash_queue_names uses one-character (letter or digit)
directory names. This is sufficient to implement a tree.
Current queue file names are longer than 1, because they have two
parts (inode number and time).
A future queue implementation may use a different organization:
one tree that combines incoming, active and deferred, with file
names that are cryptograpically unique, and without rename()
operations, but some other mechanism to keep track of state
so that we can eliminate reading directories.
Wietse
