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; 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 a system memory of said local coherency domain to indicate that said target memory block is cached, if at all, only within said local coherency domain; and in response to said coherency response indicating that said target memory block is not cached in said remote coherency domain, at least one cache hierarchy in said local coherency domain updating its respective coherency state for said target memory block from a first state indicating that said memory block may be cached in said remote coherency domain to a second state indicating that said target memory block is cached, if at all, only within said local coherency domain.