THEN CAME ASYNC
Asynchronous replication compromises some timeliness of data and as such, data integrity, for higher performance and minimal application impact on the primary system – across any distance, usually using IP networks.
With asynchronous replication, there is no connection between the primary write operation and the remote write operation. Once data has been written to the primary storage site, new writes from the application can be accepted without having to wait for confirmation that the secondary or remote storage site has successfully completed its writes.
Asynchronous replication has the advantage of speed and significantly reduces the need for network bandwidth, particularly for large files such as databases and activity logs. It is also quite flexible, allowing users to continuously replicate only the files or directories they deem business critical and worthy of immediate recovery.
Unlike synchronous replication, asynchronous replication can span any distance without negative impact from propagation delay. This means that the secondary site can be located thousands of miles away from the primary site, ensuring that the secondary data is outside any likely disaster zone.
The main disadvantage of asynchronous replication is a time lag between data being stored at the primary and remote sites. Transactions and data not replicated at the time of the disaster will be lost. In the event of an unplanned outage, data on the secondary storage may not always be current.