Dell EqualLogic PS6210XS EqualLogic Group Manager Administrator s Guide PS Ser - Page 298

Table 53. Comparing Synchronous Replication and Traditional Replication provides in-depth information about the differences between the two features. Table 53. Comparing Synchronous Replication and Traditional Replication Consideration Traditional Replication Synchronous Replication (SyncRep) Typical use case A point-in-time process that is conducted between two groups, often in geographically diverse locations. Replication provides protection against a regional disaster such as an earthquake or hurricane. Traditional replication has the advantage of providing multiple recovery points. A real-time process that keeps two identical copies of volume data in two different pools within the same PS Series group. Synchronous replication is useful for maintaining two copies of a volume's data in the same data center, or dispersed to two different facilities on the same campus or in the same metropolitan area. A disadvantage of traditional replication is that the state of the data between recovery points is unknown; if any changes are made to the volume since the last replica was created, they could be lost. An advantage of synchronous replication is that it captures a duplicate copy of every write. One disadvantage is that if an application writes bad data to the volume, the bad data is simultaneously written to both the SyncActive and SyncAlternate volumes. Recovery time If a disaster occurs in the primary group, you can If a disaster involving the active pool occurs, you promote the replica set on the secondary group to can manually switch the volume to the alternate a recovery volume. pool. After the promotion, you must reconfigure After the switch, the alternate pool becomes the initiators to discover and log in to the iSCSI target active pool and hosts the volume. now hosted by the secondary group, or switch to an alternate set of server resources that have been Host access to the volume is disrupted by the preconfigured to use the secondary group storage. switch, but iSCSI initiators do not need to be See "Impact on applications" for more information. reconfigured. Recovery point The recovery volume contains point-in-time data that is current as of the most recent replica. Replication can be scheduled to take place as frequently as once every 5 minutes. You can also restore data to the point in time when any previous replicas were created, provided that the replicas have been retained. Synchronous replication provides a single recovery point: the most recent acknowledged write to the volume. Network requirements Replication requires that the network connection between the primary and secondary groups must be able to handle the load of the data transfer and complete the replication in a timely manner. Because writes are not acknowledged until they are written to both the active and alternate pools, synchronous replication is sensitive to network latency. The network must be able to handle the load of the data transfer from the active pool to the alternate pool and complete the replication in a timely manner, or application performance could suffer. Snapshots Replication is functionally similar to snapshots, creating point-in-time copies of the volume. If the keep failback option is enabled, the group creates a "failback snapshot" on the primary group every time a replica is created. Synchronous replication creates snapshots of the volume whenever the SyncActive and SyncAlternate volumes are switched. In addition, you can schedule the creation of snapshots, or create them on demand, as you would with any other volume. 298 About Synchronous Replication