Cisco Systems OL-12518-01 manual Applications, Synchronous Replication, Asynchronous Replication

Chapter 4 FCIP over IP/MPLS Core

Applications

Figure 4-4 Using the GUI to Apply QoS

Applications

Disaster recovery and business continuance plans drive the need for solutions that protect critical business information and provide continuous access to important data in case of disaster. Disaster recovery applications are intended to replicate data to a remote backup location. The backup site can be located in the same metro area, such as New York and New Jersey, or at transcontinental distances. The more stringent requirements of business continuance emphasize real-time restoration; when disaster occurs, failover is nearly immediate, providing for faster recovery. Business continuance is put in place to protect business applications at times when downtime is not an option. Common applications for replicating and protecting critical information include synchronous and asynchronous replication and tape backup.

Synchronous Replication

Synchronous replication protects data and applications that have stringent availability requirements. Some applications, such as online trading, must be designed and implemented so that no data is lost in case of a disaster. To achieve this, transactions must be written on both the main and backup sites synchronously to keep the databases consistent. When an application writes data to disk, that data is being replicated to the remote site before a write acknowledgement is sent back to the application. The write I/O is acknowledged on the server only when a block of data has been written on both sites. Therefore, the latency introduced in the network directly affects the application performance.

To limit the impact of replication, storage array vendors are imposing distance limitations for synchronous replication. The distance is typically around 100 kilometers.

Asynchronous Replication

If a limited amount of business information can be lost in case of disaster, asynchronous replication can be used. Asynchronous replication provides very good protection, but some transactions can be lost in case of disaster. With asynchronous replication, write I/O is completed after being written on the main storage array. The server does not wait until the I/O is completed on the other storage array. There is no distance limitation and typical asynchronous replication applications can span thousands of kilometers or more. See Application Requirements, page 4-29for more on application requirements.

	Data Center High Availability Clusters Design Guide
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