Specifies the depth of directory hashing. This is used for physical filesystems that cannot handle huge numbers of files in a single directory.
When generating a filename from a binary string such as a Hash,
for a DEPTH of 
, the first groups of SIZE hexadecimal
digits are used as nested directory names.
For example, at DEPTH 
and SIZE 
, the
empty file, using an MD5 hash, would have a filename like
`d/4/1/d41d8cd98f00b204e9800998ecf8427e'.