A target data storage system performing a migration operation also responds to client requests by creating and maintaining a metadata cache for caching attributes of a user file as obtained from a source data storage system. A read-type client request is processed by (1) determining whether the metadata cache stores current attribute values, (2) if so, then satisfying the client request using the current values in the metadata cache, and (3) otherwise obtaining current attribute values from the source data storage system and satisfying the client request using the obtained current attribute values, and storing the current values in the metadata cache to be available for use in processing subsequent read-type client requests. A write-type client request is processed forwarding modified attribute values from the client request to the source data storage system for storage therein.