You can logically group instances of an OPS database into multiple instance groups. You do this by setting the INSTANCE_GROUPS parameter in the initialization parameter file. The INSTANCE_GROUPS parameter defines instance groups and assigns an instance to them. For example, when you specify the following INIT.ORA parameter for an instance, that instance will belong to groupA:
INSTANCE_GROUPS = groupA
One instance can belong to multiple instance groups at the same time. The INSTANCE_GROUPS parameter specifies the group names using a comma-separated list—for example:
INSTANCE_GROUPS = groupA, groupB
With this setting, the instance will be in groupA as well as in groupB.
The instances belonging to a particular instance group can be determined by referring to the INIT.ORA files of all the instances. Let’s consider an OPS configuration with four instances having the settings shown in Table 7.2.
Table 7-2. INSTANCE_GROUPS Settings for a Four-Instance Configuration
|
Instance |
INSTANCE_GROUPS Setting |
|---|---|
|
1 |
g12, g14, g123 |
|
2 |
g12, g23, g123 |
|
3 |
g23, g123 |
|
4 |
g14 |
The settings shown in Table 7.2 define four instance groups—g12, g14, g123, and g23 in the system—and assign instances to groups as specified. Instance group g12 is specified in the initialization parameter files of instances 1 and 2. Therefore, instance group g12 consists of those two instances. Group names do not need to reflect the instance numbers. We could have used any arbitrary name in place of g12.
Instance groups can overlap each other, meaning that two groups can share one or more instances. The INSTANCE_GROUPS parameter is static and cannot be changed while the instances are running.
Instance groups are useful for two reasons:
They allow you to summarize information returned by the GV$ views in a useful manner. The OPS_ADMIN_GROUP initialization parameter is used to facilitate this. Chapter 10, discusses this use of the GV$ views in more detail. Note that OPS_ADMIN_GROUP is obsolete in Oracle8i.
They allow you to specify which group of instances gets used for parallel execution of SQL statements. The PARALLEL_INSTANCE_GROUP initialization parameter (discussed in Chapter 13) is used for this purpose.