1. A method of data processing in a data processing system, said method comprising: scrubbing logic in a local coherency domain issuing to at least one cache hierarchy in a remote coherency domain a domain reset request that forces invalidation of any cached copy of a target memory block then held in said remote coherency domain; receiving a coherency response to said domain reset request; and in response to said coherency response indicating that said target memory block is not cached in said remote coherency domain, updating a domain indication in said local coherency domain to indicate that said target memory block is cached, if at all, only within said local coherency domain.