Use git-like hashed directories

[François Marier]

To avoid having too many files in the same directory, we should do what git does with its "objects" and pull the first two characters of the hash into a directory.

A file like this:

   /avatar/abcd1234

would therefore be stored as:

  /avatar/ab/cd1234

[Andrew Ruthven (andrew-etc)]

You could also use:

  /avatar/ab/abcd1234

Which might make things a little bit easier, especially if you need to insert another level of hashing later.