Introduction to Replication

There are many disaster recovery solutions available today. They must protect against not only day-to-day mistakes, such as users erroneously deleting volumes, but also computer viruses and site disasters.

Some solutions are time-consuming and require you to back up data and manually transport the backups to a different location. Other solutions require expensive hardware and the ability to synchronously copy data across long distances, which can adversely affect application performance.

Replication technology from EqualLogic enables you to protect data from serious failures, ranging from the destruction of a volume to a complete site disaster, with no impact on data availability or performance. Similar to a snapshot, a replica represents the contents of a volume at a specific point in time. A replica set is the entire set of complete replicas for a volume.

Replicas are stored in a group that is different from the group where the original volume resides. Data can be recovered from replicas if the volume is destroyed. Also, by separating the groups geographically, volume data is protected against a complete site disaster.

You can create replicas of an individual volume or all the volumes in a volume collection. When you create replicas of the volumes in a collection, the resulting set of replicas (one for each volume) is called a replica collection. See Creating a Replica at the Current Time and Creating a Replica Collection at the Current Time for more information.

In addition, you can set up schedules to automatically create replicas or replica collections at a specific time in the future or on a regular basis. See Managing Schedules for Snapshots and Replicas for information.

To set up replication across two PS Series groups, you must configure the groups as replication partners. The groups can be physically located in the same building or separated by some distance, depending on your needs.

Each replication partner plays a specific role in the replication of a volume. The primary group is where the volumes are located. The secondary group stores the replica set in a portion of space that it has delegated to the primary group for this purpose. The direction of the replication is outbound from the primary group and inbound to the secondary group. Mutual authentication provides security between partners.

When you create a replica, either at the current time or by using a schedule, the first replica of a volume is a complete transfer of the volume data at that point in time to the secondary group. For subsequent replicas, only the data that changed since the previous replica is transferred.

Configuring volume replication is a three-part process:

  1. Establish a replication partnership between two groups. It is important that there is adequate network bandwidth between the partners, in addition to full IP routing. Both partners must also have the necessary space for replication.
  2. Enable a volume for replication, specifying the partner that will store the replicas, the local reserve for tracking volume changes during replication, and the replica reserve on the partner for storing the replica set. If you are replicating a volume collection, you must also enable replication on the volume collection.
  3. Create a replica at the current time or set up a schedule to replicate the volume or volume collection.

Although both the primary group and the secondary group have access to volume replicas, it is recommended that the primary group administrator control the replication activity. As the "owner" of the volumes, the primary group is also considered the "owner" of the volume replicas. However, if the primary group is destroyed or if you need to recover data from the replicas, you must access the replicas from the secondary group.

For example, if the original volume is destroyed, you can log in to the secondary group and either clone a replica or promote a replica set. The two methods of recovering data differ in results and in the amount of space required in the secondary group:

The following replication administration topics describe: