A
Restrictions

This appendix documents Real Application Clusters compatibility issues and restrictions. Topics in this appendix include:

Compatibility Between Shared and Exclusive Mode

The following sections describe aspects of compatibility between shared and exclusive modes on a Real Application Clusters database:

Export and Import Utilities

The Export utility writes data from an Oracle database into operating system files, and the Import utility reads data from those files back into an Oracle database. This feature of Oracle is the same in shared or exclusive mode.

Mode Compatibility

Real Application Clusters runs with any Oracle database created in exclusive (X) mode. Each instance must have its own set of redo logs.

Oracle in exclusive mode can access a database created or modified by the Real Application Clusters software.

If Real Application Clusters allocates free space to a specific instance, then that space might not be available for inserts for a different instance in exclusive mode. All data in the allocated extents is, of course, always available.

Restrictions

The following sections describe restrictions of Real Application Clusters:

Maximum Number of Blocks Allocated at a Time

The !blocks option of the GC_FILES_TO_LOCKS parameter enables you to control the number of blocks available for use within a free list group. You can use !blocks to specify the rate that blocks are allocated within an extent, up to 255 blocks at a time.

Restrictions in Cluster Database Mode

Oracle running multiple instances in cluster database mode supports all the functionality of Oracle in exclusive mode, except as noted in the following sections.

Restricted SQL Statements

In cluster database mode, the following operations are not supported:

Creating a database (CREATE DATABASE)
Creating a control file (CREATE CONTROLFILE)
Switching the database's archiving mode (the ARCHIVELOG and NOARCHIVELOG options of ALTER DATABASE)

To perform these operations, shut down all instances and start up one instance in exclusive mode.

Maximum Number of Datafiles

The number of datafiles supported by Oracle is operating system-specific. Within this limit, the maximum number allowed depends on the values used in the CREATE DATABASE command. This in turn is limited by the physical size of the control file. This limit is the same in cluster database mode as in exclusive mode, but the additional instances of Real Application Clusters restrict the maximum number of files more than a single instance system.

See Also:

Oracle9i SQL Reference and your Oracle operating system-specific documentation

Sequence Number Generators

Real Application Clusters does not support CACHE ORDER combination of options for sequence number generators in cluster database mode. Sequences created with both of these options are ordered but not cached when running in Real Application Clusters.

Free Lists with Import and Export Utilities

The Export utility does not preserve information about multiple free lists and free list groups. The metadata of the table where it is imported contains the free list and free list group information that is henceforth associated with the data blocks.

Therefore, if you use the Export and Import utilities to back up and restore your data, then it will be difficult to import the data so that it is partitioned again.

ARestrictions