Important things to know about nodes and clusters.
When the iASP is switched from SYSA to SYSB it should have the same iASP name, iASP number, disk unit numbers and virtual address. This is achieved by making cluster nodes as member of a construct called Device domain. Device domain also prevents conflicts that can cause an attempt to switch the iASP to fail. So, after creation of the cluster and enrollment of nodes in to it, you add nodes to a device domain (ADDDEVDMNE). Note that a cluster node can belong to only one device domain.
Let us move on now to Cluster Resource Groups. A Cluster Resource Group (CRG) is an IBMi System Object that is used to manage high availability of resources in a cluster.
IBM i cluster resource services provide the ability to create specific type of CRGs like application, data, device and peer. The differences are noted below.
Each of the supported IBM i cluster CRGs has two common elements: a recovery domain and an exit program. The exit program invokes actions when the CRG detects cluster wide events such as node failure or application errors. The exit program is optional for a resilient Device CRG but is required for the other CRG types.
The CRG manages resource availability across a subset of nodes within the cluster called a recovery domain. Within recovery domain you have a primary node and one or more backup nodes. Primary Node and Backup nodes are roles within the recovery domain. Primary Node has access to switchable resources. The role of nodes changes during switchover or failover. All nodes that will be in the recovery domain for a device CRG must be in the same device domain.
Tasks to perform when creating a switchable cluster:
How does switching work?
If the iASP is varied on, cluster resource services will try to end jobs having the iASP in its namespace and then vary off the iASP on SYSA.
If the device CRG was set to vary on the iASP on SYSB, the cluster resource services will submit a batch job QCSTVRYDEV on SYSB for varying on the iASP.
If the switchover is successful the device CRG status changed from Switchover Pending to Active.
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