Oracle 10g Automated Storage Management (ASM)

Oracle RAC Cluster Tips by Burleson Consulting

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Automated Storage Management (ASM) divides files into 1 MB extents and spreads the extents for each file evenly across all of the disks in a disk group. ASM does not use a mathematical function to track the placement of each extent; it uses pointers to record extent location. This allows ASM to move individual extents of a file when the disk group configuration changes without having to move all extents to adhere to a formula based on the number of disks.

As stated above, for normal datafiles, ASM uses 1 megabyte extents. For log files that require low latency, ASM provides fine-grained (128k) striping to allow larger I/Os to be split and processed in parallel by multiple disks. The DBA decides at creation time whether or not to use fine-grained striping. File-type specific templates in the disk group determine the default behavior.

Automatic Storage Management?s native mirroring mechanism is an option that can be set when creating a disk group. The redundancy protection for that group can be specified and, if the DBA prefers to use an external mirroring system or has no need for fault-tolerance, the DBA can set the redundancy to external, in which case ASM will not mirror at all. Unless otherwise specified, redundancy defaults to normal, in which case ASM mirrors each 1 MB extent.

For the highest level of fault tolerance, high redundancy can be specified, which provides three-way mirroring.

Server Control Utility

To manage the RAC database and its instances, Oracle provided a utility called the Server Control Utility (SRVCTL) in version 9i. This replaced the earlier utility opsctl which was used in the parallel server.

The Server Control Utility is a single point of control between the Oracle Intelligent agent and each node in the RAC system. The SRVCTL communicates with the global daemon service (GSD) and resides on each of the nodes. The SRVCTL gathers information from the database and instances and acts as an intermediary between nodes and the Oracle Intelligent agent.

When using the SRVCTL to perform configuration operations on a cluster, the SRVCTL stores configuration data in the Server Management (SRVM) configuration repository. The SRVM includes all the components of Enterprise Manager such as the Intelligent Agent, the Server Control Utility (SRVCTL), and the Global Services Daemon. Thus, the SRVCTL is one of the SRVM Instance Management Utilities. The SRVCTL uses SQL*Plus internally to perform stop and start activities on each node.

For the SRVCTL to function, the Global Services Daemon (GSD) should be running on the node. The SRVCTL performs mainly two types of administrative tasks: Cluster Database Tasks and Cluster Database Configuration Tasks.

SRVCTL Cluster Database tasks include:

* Starts and stops cluster databases.

* Starts and stops cluster database instances.

* Starts and stops listeners associated with a cluster database instance.

* Obtains the status of a cluster database instance.

* Obtains the status of listeners associated with a cluster database.

* Start and stop ASM instances.

* Manage disk groups in an ASM instance.

* SRVCTL Cluster Database Configuration tasks include:

* Adds and deletes cluster database configuration information.

* Adds an instance to, or deletes an instance from a cluster database.

* Renames an instance name within a cluster database configuration.