...
The PairTree algorithm is the default method for generating identifiers in Fedora 4 default. This method will be used for the migration and a for new object to address the performance issue about limiting the number of children under a single resource (Performance). As for the legacy PID, it will be stored as a property of the node as mentioned above. For example, refer Refer to the URL structures on Data Model section for example.
Indexing strategies (GSearch, RI-Search vs. F4 approaches)
...