Replication – The copying at a block level of data in a source volume or file system to a target system. When first set up there is an initial synchronisation between the source and target systems and then every block-level change on the source system will be copied to the target system. It is possible to have file-level replication but that is inefficient as a single block-level change will trigger an entire file replication.
A synchronous replication system replicates any changes to the target system before telling the source system that the data has been written. Asynchronous replication creates a queue of changes and carries them out in order but not necessarily straight away when a change is registered in the target system. This does not affect the performance of the target system but the replicated copy will be out of step with the source until the queue of changes is empty and no more changes in the source system have taken place.