XRC also uses the Data Mover host program. After the source and target devices have been synchronized, each modification to the source device is monitored and kept in a temporary repository. It is then read by the Data Mover, and merged with data from other devices and storage subsystems into a consistency group, which the Data Mover transfers to the secondary site. The wisdom of this function is in keeping a consistent image of all devices being monitored with minimal impact on the performance of applications using the source device. PPRC provides disaster recovery on a device basis for medium distances.
Data transfer is performed by the storage subsystems with no host assistance.
The storage subsystem containing the source device communicates directly with the storage subsystem containing the secondary device. The devices are first synchronized, and then each modification to a source device is transferred to the secondary device synchronously with the application operation. The wisdom of this function is in managing and transferring the data synchronously with minimal impact on the application using the source device. To achieve the effect of an instant copy, the storage subsystem manipulates a set of data structures so that unmodified data serves both the source and target devices, while the original image of modified data is transferred to the target device. Optionally, a background task can copy all original source data to the target.