Throttling an asynchronous remote copying system

A storage control system comprising: one or more I/O processors for receiving requests to write data; a batch controller for selecting one or more write requests to be placed in a first batch; a first server for transmitting the first batch to a second server in a remote copy pair system; one or more I/O handlers for writing the first batch to completion on a data storage medium, in response to receiving permission from the second server; and one or more clients for communicating with the I/O processors and the I/O handlers, wherein the permission is granted when the second server is notified by the I/O handlers that previous write requests have been completed, wherein the first server is responsive to the completion of the first batch.