HA clusters require a shared storage and cluster file system for active/passive & active/active architectures respectively. Any application can be made highly available only by providing concurrent access to data of the application, which requires a SAN shared storage.
Concurrent access to data can be provided by mirroring data between nodes and this eliminates the need for expensive SAN shared storage.
In real time. Replication occurs continuously, while applications modify the data on the device.
The applications that store their data on the mirrored device are oblivious of the fact that the data is in fact stored on several computers.
With synchronous mirroring, a writing application is notified of write completion only after the write has been carried out on both computer systems.
Asynchronous mirroring means the writing application is notified of write completion when the write has completed locally, but before the write has propagated to the peer system.
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