Oracle RAC extends Oracle Database so that you can store, update, and efficiently retrieve data from multiple instances on different servers at the same time. Oracle RAC provides the software that allows the servers to work together in what is called a cluster. The physical structures that make up the database must reside on shared storage that is accessible from all servers that are part of the cluster. Each server in the cluster runs the Oracle RAC software.



Single-instance Oracle databases have a one-to-one relationship between datafiles and the instance. Oracle RAC environments, however, have a one-to-many relationship between datafiles and instances. In Oracle RAC environments, multiple cluster database instances form a single database. The instances can be on different servers, referred to as hosts or nodes. The combined processing power of the multiple servers provides greater throughput and scalability than is available from a single server.

Each cluster database instance in an Oracle RAC cluster uses its own memory structures and background processes. Oracle RAC uses Cache Fusion to synchronize the data stored in the buffer cache of each cluster database instance. Cache Fusion moves current data blocks (which reside in memory) between database instances, rather than having one database instance write the data blocks to disk and requiring another database instance to reread the data blocks from disk. When a data block located in the buffer cache of one instance is required by another instance, Cache Fusion transfers the data block directly between the instances using the interconnect, enabling the Oracle RAC database to access and modify data as if the data resided in a single buffer cache.

The Oracle RAC infrastructure is also a key component for implementing the Oracle enterprise grid computing architecture. Having multiple cluster database instances accessing a single database prevents the server from being a single point of failure. Any packaged or custom application that ran well on a single-instance Oracle database will perform well on an Oracle RAC database without requiring code changes.

About Oracle Automatic Storage Management

With Oracle RAC, each instance must have access to the datafiles and recovery files for the Oracle RAC database. Using Automatic Storage Management (ASM) is an easy way to satisfy this requirement.

ASM is an integrated, high-performance database file system and disk manager. ASM is based on the principle that the database should manage storage instead of requiring an administrator to do it. ASM eliminates the need for you to directly manage potentially thousands of Oracle database files.

ASM groups the disks in your storage system into one or more disk groups. You manage a small set of disk groups and ASM automates the placement of the database files within those disk groups.

ASM provides the following benefits:

Striping

ASM spreads data evenly across all disks in a disk group to optimize performance and utilization. This even distribution of database files eliminates the need for regular monitoring and I/O performance tuning.

Mirroring

ASM can increase data availability by optionally mirroring any file. ASM mirrors at the file level, unlike operating system mirroring, which mirrors at the disk level. Mirroring means keeping redundant copies, or mirrored copies, of each extent of the file, to help avoid data loss caused by disk failures. The mirrored copy of each file extent is always kept on a different disk from the original copy. If a disk fails, ASM can continue to access affected files by accessing mirrored copies on the surviving disks in the disk group.

Online storage reconfiguration and dynamic rebalancing

ASM permits you to add or remove disks from your disk storage system while the database is operating. When you add a disk, ASM automatically redistributes the data so that it is evenly spread across all disks in the disk group, including the new disk. The process of redistributing data so that it is also spread across the newly added disks is known as rebalancing. It is done in the background and with minimal impact to database performance.

Managed file creation and deletion

ASM further reduces administration tasks by enabling files stored in ASM disk groups to be managed by Oracle Database. ASM automatically assigns file names when files are created, and automatically deletes files when they are no longer needed.

ASM is implemented as a special kind of Oracle instance, with its own System Global Area and background processes. The ASM instance is tightly integrated with the database instance. Every server running one or more database instances that use ASM for storage has an ASM instance. In an Oracle RAC environment, there is one ASM instance for each node, and the ASM instances communicate with each other on a peer-to-peer basis. Only one ASM instance is required for each node regardless of the number of database instances on the node.

Oracle recommends that you use ASM for your database file storage, instead of raw devices or the operating system file system. However, databases can have a mixture of ASM files and non-ASM files.

Tools for Installing, Configuring and Managing Oracle RAC

The following is a description of the tools used for installing, configuring, and managing an Oracle RAC database:

Oracle Universal Installer (OUI)

OUI installs the Oracle Clusterware and the Oracle Database software with Oracle RAC. After you configure the nodes that you want to use in your cluster, OUI installs the Oracle software on the specified nodes using a network connection.

Cluster Verification Utility (CVU)

The CVU is a command-line tool that you can use to verify a range of cluster and Oracle RAC components such as shared storage devices, networking configurations, system requirements, and Oracle Clusterware, as well as operating system groups and users. You can use the CVU for preinstallation as well as postinstallation checks of your cluster environment. The CVU is especially useful during preinstallation and during installation of Oracle Clusterware and Oracle RAC components. OUI runs the CVU after the Oracle Clusterware installation to verify your environment.

Oracle Enterprise Manager

Oracle Enterprise Manager has both the Database Control and Grid Control graphical user interfaces (GUIs) for managing single- instance and Oracle RAC environments.

Server Control (SRVCTL)

SRVCTL is a command-line interface that you can use to manage an Oracle RAC database from a single point. You can use SRVCTL to start and stop the database and instances, and to delete or move instances and services. You can also use SRVCTL to add services and manage configuration information. You use SVRCTL to start and stop a group of applications that includes virtual IP addresses, Listeners, Oracle Notification Services, node-level applications, and Oracle Enterprise Manager agents (for maintenance purposes).

Cluster Ready Services Control (CRSCTL)

CRSCTL is a command-line tool that you can use to manage Oracle Clusterware. You can use CRSCTL to start and stop Oracle Clusterware and to determine the current status of your Oracle Clusterware installation.

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