Using Storage-Based Data Replication Within a Cluster

Storage-based data replication uses software installed on the storage device to manage the replication within a cluster or a campus cluster. Such software is specific to your particular storage device, and is not used for disaster recovery. Refer to the documentation that shipped with your storage device when configuring storage-based data replication.

Depending on the software you use, you can use either automatic or manual failover with storage-based data replication. Sun Cluster supports both manual and automatic failover of the replicants with Hitachi TrueCopy, Hitachi Universal Replicator, and EMC SRDF software.

This section describes storage-based data replication as used in a campus cluster. Figure 4–1 shows a sample two-room configuration where data is replicated between two storage arrays. In this configuration, the primary storage array is contained in the first room, where it provides data to the nodes in both rooms. The primary storage array also provides the secondary storage array with data to replicate.

Figure 4–1 illustrates that the quorum device is on an unreplicated volume. A replicated volume cannot be used as a quorum device.

Figure 4–1 Two-Room Configuration With Storage-Based Data Replication

Storage-based data replication with Hitachi TrueCopy or Hitachi Universal Replicator can be performed synchronously or asynchronously in the Sun Cluster environment, depending on the type of application you use. If you want to perform automatic failover in a campus cluster, use TrueCopy synchronously. Storage-based synchronous replication with EMC SRDF is supported with Sun Cluster; asynchronous replication is not supported for EMC SRDF.

Do not use EMC SRDF's Domino mode or Adaptive Copy mode. Domino mode makes the local and target SRDF volumes unavailable to the host when the target is unavailable. Adaptive Copy mode is generally used for data migrations and data center moves and it not recommended for disaster recovery.

Do not use the Data or Status modes in Hitachi TrueCopy or Hitachi Universal Replicator. If the secondary storage device fails, you might experience problems writing to the primary storage device.

Requirements and Restrictions When Using Storage-Based Data Replication Within a Cluster

To ensure data integrity, use multipathing and the proper RAID package. The following list includes considerations for implementing a cluster configuration that uses storage-based data replication.

• Node-to-node distance is limited by the Sun Cluster Fibre Channel and interconnect infrastructure. Contact your Sun service provider for more information about current limitations and supported technologies.

• Do not configure a replicated volume as a quorum device. Locate any quorum devices on a shared, unreplicated volume or use the quorum server.

• Ensure that only the primary copy of the data is visible to cluster nodes. Otherwise, the volume manager might try to simultaneously access both primary and secondary copies of the data. Refer to the documentation that was shipped with your storage array for information about controlling the visibility of your data copies.

• EMC SRDF, Hitachi TrueCopy, and Hitachi Universal Replicator allow the user to define groups of replicated devices. Each replication device group requires a Sun Cluster device group with the same name.

• Particular application-specific data might not be suitable for asynchronous data replication. Use your understanding of your application's behavior to determine how best to replicate application-specific data across the storage devices.

• If configuring the cluster for automatic failover, use synchronous replication.

  For instructions on configuring the cluster for automatic failover of replicated volumes, see Administering Storage-Based Replicated Devices.

• Oracle Real Application Clusters (RAC) is not supported with SRDF, Hitachi TrueCopy, and Hitachi Universal Replicator when replicating within a cluster. Nodes connected to replicas that are not currently the primary replica will not have write access. Any scalable application that requires direct write access from all nodes of the cluster cannot be supported with replicated devices.

• Veritas Cluster Volume Manager (CVM) and Solaris Volume Manager (SVM) OBAN cluster for Sun Cluster software are not supported.

• Do not use the Domino mode or Adaptive Copy mode in EMC SRDF. See Using Storage-Based Data Replication Within a Cluster for more information.

• Do not use the Data mode or Status mode in Hitachi TrueCopy or Hitachi Universal Replicator. See Using Storage-Based Data Replication Within a Cluster for more information.

Manual Recovery Concerns When Using Storage-Based Data Replication Within a Cluster

As with all campus clusters, those clusters that use storage-based data replication generally do not need intervention when they experience a single failure. However, if you are using manual failover and you lose the room that holds your primary storage device (as shown in Figure 4–1), problems arise in a two–node cluster. The remaining node cannot reserve the quorum device and cannot boot as a cluster member. In this situation, your cluster requires the following manual intervention:

1. Your Sun service provider must reconfigure the remaining node to boot as a cluster member.

2. You or your Sun service provider must configure an unreplicated volume of your secondary storage device as a quorum device.

3. You or your Sun service provider must configure the remaining node to use the secondary storage device as primary storage. This reconfiguration might involve rebuilding volume manager volumes, restoring data, or changing application associations with storage volumes.

Best Practices When Using Storage-Based Data Replication

When setting up device groups that use the Hitachi TrueCopy or Hitachi Universal Replicator software for storage-based data replication, observe the following practices:

• Use synchronous replication to avoid the possibility of lost data if the primary site fails.

• A one-to-one relationship should exist between the Sun Cluster global device group and the TrueCopy replication group defined in the horcm configuration file. This allows both groups to move from node to node as a single unit.

• Global file system volumes and failover file system volumes cannot be mixed in the same replicated device group because they are controlled differently. Global file systems are controlled by a Device Configuration System (DCS), while failover file system volumes are controlled by HAS+. The primary for each could be a different node, causing conflicts on which node should be the replication primary.

• All RAID manager instances should be up and running at all times.

When using EMC SRDF software for storage-based data replication, use dynamic devices instead of static devices. Static devices require several minutes to change the replication primary and can impact failover time.