The aim of online data replication is not only to guarantee a second copy of data identical to that of production, but to guarantee the possible reconstruction and exploitation of copies which have survived the disaster. Files system or database replication tools should reflect the integrity of the data replicated so that these can be used in case of disaster. These tools should integrate mechanisms called Write Ordering or of Crash Consistency. Replication technologies can be classified in two categories:

• Point-In-Time Copy (Snapshot)
• Real Time Copy