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Introduction to Oracle RAC

    cluster comprises multiple interconnected computers or servers that appear as if they are one server to end users and applications. Oracle RAC enables you to cluster an Oracle database. Oracle RAC uses Oracle Clusterware for the infrastructure to bind multiple servers so they operate as a single system.

    Oracle Clusterware is the only clusterware that you need for most platforms on which Oracle RAC operates. You can also use clusterware from other vendors if the clusterware is certified for Oracle RAC.

    An Oracle RAC database can have up to 100 instances, all of which access one database. All database instances must use the same interconnect, which can also be used by Oracle Clusterware.

    Oracle RAC databases differ architecturally from noncluster Oracle databases in that each Oracle RAC database instance also has:

    • At least one additional thread of redo for each instance
    • An instance-specific undo tablespace

    The combined processing power of the multiple servers can provide greater throughput and Oracle RAC scalability than is available from a single server.

    Figure below shows how Oracle RAC is the Oracle Database option that provides a single system image for multiple servers to access one Oracle database. In Oracle RAC, each Oracle instance must run on a separate server.

    Description of Figure 1-1 follows

    Oracle RAC is a unique technology that provides high availability and scalability for all application types. The Oracle RAC infrastructure is also a key component for implementing the Oracle enterprise grid computing architecture. Having multiple instances access a single database prevents the server from being a single point of failure. Oracle RAC enables you to combine smaller commodity servers into a cluster to create scalable environments that support mission critical business applications. Applications that you deploy on Oracle RAC databases can operate without code changes.

    Oracle Clusterware for Oracle RAC

    Oracle Clusterware is designed for, and tightly integrated with, Oracle RAC. You can use Oracle Clusterware to manage high-availability operations in a cluster. When you create an Oracle RAC database using any of the management tools, the database is registered with and managed by Oracle Clusterware, along with the other required components such as the Virtual Internet Protocol (VIP) address, the Single Client Access Name (SCAN), the SCAN listener, Oracle Notification Service, and the Oracle Net listeners. These resources are automatically started when Oracle Clusterware starts the node and automatically restarted if they fail. The Oracle Clusterware daemons run on each node.

    Anything that Oracle Clusterware manages is known as a CRS resource. A CRS resource can be a database, an instance, a service, a listener, a VIP address, or an application process. Oracle Clusterware manages CRS resources based on the resource’s configuration information that is stored in the Oracle Cluster Registry (OCR). You can use SRVCTL commands to administer any Oracle-defined CRS resources. Oracle Clusterware provides the framework that enables you to create CRS resources to manage any process running on servers in the cluster which are not predefined by Oracle. Oracle Clusterware stores the information that describes the configuration of these components in OCR.

    Overview of Oracle RAC Architecture and Processing

    At a minimum, Oracle RAC requires Oracle Clusterware software infrastructure to provide concurrent access to the same storage and the same set of data files from all nodes in the cluster, a communications protocol for enabling interprocess communication (IPC) across the nodes in the cluster, enable multiple database instances to process data as if the data resided on a logically combined, single cache, and a mechanism for monitoring and communicating the status of the nodes in the cluster.

    Understanding Cluster-Aware Storage Solutions

    An Oracle RAC database is a shared everything database. All data files, control files, SPFILEs, and redo log files in Oracle RAC environments must reside on cluster-aware shared disks, so that all of the cluster database instances can access these storage components. Because Oracle RAC databases use a shared everything architecture, Oracle RAC requires cluster-aware storage for all database files.

    In Oracle RAC, the Oracle Database software manages disk access and is certified for use on a variety of storage architectures. It is your choice how to configure your storage, but you must use a supported cluster-aware storage solution. Oracle Database provides the following file storage options for Oracle RAC:

    • Oracle Automatic Storage Management (Oracle ASM)Oracle recommends this solution to manage your storage.
    • A certified cluster file system, including OCFS2 and Oracle Cluster File System (OCFS for Windows)OCFS2 is available for Linux, and OCFS for Windows is available for Windows platforms. However you may optionally use a third-party cluster file system or cluster-aware volume manager that is certified for Oracle RAC.
    • Certified network file system (NFS) file servers
    Connecting to Oracle Database Using Services and VIP Addresses

    Users can access an Oracle RAC database using a client/server configuration or through one or more middle tiers, with or without connection pooling. Users can be database administrators, developers, application users, power users, such as data miners who create their own searches, and so on.

    The interconnect network is a private network that connects all of the servers in the cluster. The interconnect network uses a switch (or multiple switches) that only the nodes in the cluster can access. Configure User Datagram Protocol (UDP) on a Gigabit Ethernet for your cluster interconnect. On Linux and UNIX systems, you can configure Oracle Clusterware to use either the UDP or Reliable Data Socket (RDS) protocols. Windows clusters use the TCP protocol. Crossover cables are not supported for use with Oracle Clusterware interconnects.

    If you use Network Attached Storage (NAS), then you are required to configure a second private network. Access to this network is typically controlled by the vendor’s software. The private network uses static IP addresses.

    Oracle RAC Software Components

    Oracle RAC databases generally have two or more database instances that each contain memory structures and background processes. An Oracle RAC database has the same processes and memory structures as a noncluster Oracle database and additional processes and memory structures that are specific to Oracle RAC. Any one instance’s database view is nearly identical to any other instance’s view in the same Oracle RAC database; the view is a single system image of the environment.

    Each instance has a buffer cache in its System Global Area (SGA). Using Cache Fusion, Oracle RAC environments logically combine each instance’s buffer cache to enable the instances to process data as if the data resided on a logically combined, single cache.

    To ensure that each Oracle RAC database instance obtains the block that it requires to satisfy a query or transaction, Oracle RAC instances use two processes, the Global Cache Service (GCS) and the Global Enqueue Service (GES). The GCS and GES maintain records of the statuses of each data file and each cached block using a Global Resource Directory (GRD). The GRD contents are distributed across all of the active instances, which effectively increases the size of the SGA for an Oracle RAC instance.

    After one instance caches data, any other instance within the same cluster database can acquire a block image from another instance in the same database faster than by reading the block from disk. Therefore, Cache Fusion moves current blocks between instances rather than re-reading the blocks from disk. When a consistent block is needed or a changed block is required on another instance, Cache Fusion transfers the block image directly between the affected instances. Oracle RAC uses the private interconnect for interinstance communication and block transfers. The GES Monitor and the Instance Enqueue Process manage access to Cache Fusion resources and enqueue recovery processing.

    Oracle RAC Background Processes

    The GCS and GES processes, and the GRD collaborate to enable Cache Fusion. The Oracle RAC processes and their identifiers are as follows:

    • ACMS: Atomic Controlfile to Memory Service (ACMS)In an Oracle RAC environment, the ACMS per-instance process is an agent that contributes to ensuring a distributed SGA memory update is either globally committed on success or globally aborted if a failure occurs.
    • GTX0-j: Global Transaction ProcessThe GTX0-j process provides transparent support for XA global transactions in an Oracle RAC environment. The database autotunes the number of these processes based on the workload of XA global transactions.
    • LMON: Global Enqueue Service MonitorThe LMON process monitors global enqueues and resources across the cluster and performs global enqueue recovery operations.
    • LMD: Global Enqueue Service DaemonThe LMD process manages incoming remote resource requests within each instance.
    • LMS: Global Cache Service ProcessThe LMS process maintains records of the data file statuses and each cached block by recording information in a Global Resource Directory (GRD). The LMS process also controls the flow of messages to remote instances and manages global data block access and transmits block images between the buffer caches of different instances. This processing is part of the Cache Fusion feature.
    • LCK0: Instance Enqueue ProcessThe LCK0 process manages non-Cache Fusion resource requests such as library and row cache requests.
    • RMSn: Oracle RAC Management Processes (RMSn)The RMSn processes perform manageability tasks for Oracle RAC. Tasks accomplished by an RMSn process include creation of resources related to Oracle RAC when new instances are added to the clusters.
    • RSMN: Remote Slave Monitor manages background slave process creation and communication on remote instances. These background slave processes perform tasks on behalf of a coordinating process running in another instance.

    Overview of Automatic Workload Management

    Oracle Database with Oracle RAC includes many features that can enhance automatic workload management, such as connection load balancing, fast connection failover, the load balancing advisory, and runtime connection load balancing. Automatic workload management provides the greatest benefits to Oracle RAC environments.

    Automatic workload management includes the following components:

    • High Availability Framework: The Oracle RAC high availability framework enables Oracle Database to always maintain components in a running state. Oracle high availability implies that Oracle Clusterware monitors and restarts critical components if they stop, unless you override the restart processing.
    • Single Client Access Name (SCAN): A single network name and IP addresses defined either in your DNS or GNS that all clients should use to access the Oracle RAC database. With SCAN, you are no longer required to modify your clients when changes occur to the cluster configuration. SCAN also allows clients to use an Easy Connect string to provide load balancing and failover connections to the Oracle RAC database.Note:SCAN is required regardless of whether you use GNS. If you use GNS, then Oracle automatically creates the SCAN. If you do not use GNS, then you must define the SCAN in DNS.
    • Load Balancing Advisory: This is the ability of the database to provide information to applications about the current service levels being provided by the database and its instances. Applications can take advantage of this information to direct connection requests to the instance that provides the application request with the best service quality to complete the application’s processing.
    • Services: Services are entities that you can define in Oracle RAC databases. Services enable you to group database workloads and route the work to the optimal instances that are assigned to process the service.
    • Server Pools: Server pools enable the CRS Administrator to create a policy which defines how Oracle Clusterware allocates resources.
    • Connection Load Balancing: Oracle Net Services provides connection load balancing for database connections. Connection load balancing occurs when the connection is created. Connections for a given service are balanced across all of the running instances that offer the service. When you enable the load balancing advisory, the listener uses the load balancing advisory for connection load balancing.