A method for managing a data storage system includes maintaining a record predictive of locations to which data are to be written on the primary storage subsystem by a host processor. Upon receiving data from the host processor at the primary storage subsystem to be written to a specified location on the first non-volatile storage media, if the specified location is not included in the record, the record is updated responsively to the specified location. 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, if the specified location was not included in the record, responsively to updating the record.