(iSeries)

High availability: continuous application data access

Without continuous access to the application data, the application servers cannot process client requests. The product supports various methods of maintaining data availability.

If our data resides on IBM i servers, we can use one of the following options to achieve higher availability for our data:

Data replication

To ensure the availability of all components of the application server environment, we can use data replication to create a backup copy of the application data. Data replication uses clustering, remote journaling, and third-party data replication software to maintain two physically separate copies of application data. Data replication can provide disaster recovery capabilities for the databases because the systems in the cluster can be geographically separated.

Clustering provides the basis for communication between two or more servers. This communication is necessary to back up the data on a physically separate machine.

Remote journaling creates a copy of the application data to assure hot backup. There are two types of remote journaling:

• Synchronous remote journaling, where data is written to both the primary and backup databases simultaneously. This type of journaling ensures that no entries are lost in the event of a system failure. However, it can negatively impact performance, because it requires the application to wait while data is written to both databases.
• Asynchronous remote journaling, where data is written to the primary database directly from the application. This type of journaling, enables the application to continue processing client requests while data is copied to the backup system. It does not impact application performance, but the most recent entries might be lost if a failure occurs.

Switchable disk

Switchable disk uses clustering, local journaling, and Independent Auxiliary Storage Pool (IASP) to ensure the availability of application data. The application data is stored in an IASP, and if a failure occurs, the IASP can be switched to another node.

The advantage of switchable disk is that no data replication occurs. Therefore, we do not have to worry about data synchronization. However, the database becomes a single point of failure. In addition, the backup node must be physically close to the primary node. Therefore, this configuration does not provide disaster recovery.

Switchable disk uses the following components and services:

• A server cluster. We must have the High Availability Switchable Resources function (5761-SS1 or 5770-SS1 Option 41) installed on all of the nodes in your cluster.
• Clustering. When we use switchable disk, clustering provides:

  • The services required to switch the IASP.
  • Wellness monitoring of backend database partitions.

• Local journaling. Switchable disk uses local journaling to preserve database transaction boundaries.

Switchable disk is best utilized in a multiple cell topology. that includes these features:

• All of the cells process client requests as described in multiple cell topology.

• One of the cells hosts the primary database system, which receives data from application servers on the other cells.

• The primary database system connects to an IASP.

• One of the other cells hosts the secondary database system, which is inactive unless there is a failure in the primary system. If the primary system fails, application data is directed to the secondary database system in this other cell, and the IASP connects to the secondary system.

Related:

Core group transports