HP StorageWorks 2/140 FW 08.01.00 McDATA Products in a SAN Environment Plannin - Page 180

Implementing SAN Internetworking Solutions 4 • Synchronous remote data replication (RDR/S) - This operational mode ensures a remote data copy (identical to the primary copy) is created at the time the primary data is created. An update operation does not complete until confirmed at both the primary and mirrored sites. An incomplete operation rolls back at both locations, ensuring the remote copy is a mirror image of the primary copy. RDR/S is synonymous with disk mirroring. The advantage to using RDR/S is quick data recovery. Operation at a remote, mirrored site begins immediately if operation at the primary site is disrupted. The problem with RDR/S is distance limitation. Although propagation of laser light pulses can theoretically extend to infinity, latency is an issue because propagation delays lengthen with increased link distance. These delays adversely impact performance by forcing an application to wait for confirmation of I/O operation at local and remote sites. This means RDR/S operation is distance-limited, depending on application response time tolerance and other factors. RDR/S is ideal for shorter metropolitan distances and real-time disk mirroring and is an appropriate BC/DR solution for enterprises requiring fast data recovery, minimal data loss, and protection against database integrity problems. • Asynchronous remote data replication (RDR/A) - This operational mode does not require a response indicating completion of a remote transaction before local I/O operations resume. Replication software on the remote storage array controller ensures data is successfully written to the remote site. Standard disk backup and tape vaulting are RDR/A operations. RDR/A may lose data transactions during an unplanned failover to a remote site. However, after post-outage transaction logs are applied to the remote data image, operations can usually resume. Catastrophic events can occur anywhere. However, it is unlikely an event will span a large geographical area or two exclusive events will simultaneously occur in two locations. Therefore, to protect critical data (required for business continuance), it is prudent to replicate the data at a remote site thousands of miles from the primary site. Because there is no propagation delay involved in confirming remote transactions, RDR/A can span virtually any geographical distance and is ideal for long-distance BC/DR applications. 4-38 McDATA Products in a SAN Environment - Planning Manual