The Differential Synchronization alghoritm does not instruct anyhow how the data should be stored.
Considering the alghoritm requires the server to store:
a) backup shadows (associated with a given client and server version document version)
b) shadows (associated with both, client and server version)
and versions are specific to client-server channel,
the alghoritm is in its design very memory inefficient.
As a solution one may store backup (and potentially shadow) documents and share them between several clients as suggested in: