Switchable disk
Switchable disk uses an independent auxiliary storage pool (IASP) to store application data. In the event of a failure, the IASP can be switched from the failed WebSphere Application Server node to another node.
The advantage of switchable disk is that there is not a replica of the data. When a replica is created, there is a risk that it is not synchronized correctly. However, the database becomes a single point of failure. In addition, the backup node must be physically close to the primary node. Because of the physical proximity of the nodes, this configuration does not provide disaster recovery.
Switchable disk uses these components and services:
An iSeries server cluster
You must install OS/400 V5R2 or higher and HA Switchable Resources (5722-SS1 Option 41) on all of the iSeries nodes in your cluster.
The machines that host WebSphere Application Server can be at V5R1 if you use the IBM Toolbox for Java JDBC driver and add a V5R2 version of the jt400.jar file to your application server classpath.
iSeries clustering
When you use switchable disk, iSeries clustering provides these services:
- The services required to switch 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 with a multiple cell topology. For example, a topology with two cells, named Cell1 and Cell2, can include these features:
- Both cells process client requests as described in Multiple cell topology.
- Cell1 hosts the primary database system, which receives data from application servers on both cells.
- The primary database system connects to IASP.
- Cell2 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 Cell2, and IASP connects to the secondary system.