According to method of data processing in a multiprocessor data processing system, in response to a processor request to modify a target granule of a target cache line of data containing multiple granules, a processing unit originates on an interconnect of the multiprocessor data processing system a data-claim-partial request that requests permission to promote only the target granule of the target cache line to a unique copy with an intent to modify the target granule. In response to a combined response to the data-claim-partial request indicating success (the combined response representing a system-wide response to the data-claim-partial-request), the processing unit promotes only the target granule of the target cache line to a unique copy by updating a coherency state of the target granule and retaining a coherency state of at least one other granule of the target cache line.