A method for managing a data storage system includes maintaining a record on a secondary storage subsystem, which is predictive of locations to which data are to be written on a primary storage subsystem by a host processor. Upon receiving at the primary storage subsystem, from a host processor, the data to be written to a specified location, if the specified location is not included in the record, the primary storage subsystem sends to the secondary storage subsystem so as to cause the secondary storage subsystem to update the record. The primary storage subsystem signals the host processor that the data have been stored in the data storage system responsively to receiving the data and to receiving an acknowledgment from the secondary storage subsystem indicating that the record has been updated.