(the answer to this might be completely different for DDS on AWS - e.g., a massive location-resolving key-value store.)
Given a relative file path in a METS file (e.g., a JP2 or ALTO file), the DDS needs to resolve the S3 location of that file.
The simple way to do this is load the storage manifest, find the entry corresponding to the path in METS, and deduce the S3 URI. But that would be very inefficient.
So the DDS caches a StorageMap for each storage manifest, which is this object, serialised:
[Serializable]