Tuesday, February 5, 2008

Cluster in Brief

Major Oracle Clusterware components

  • The Oracle Clusterware Software Components

Cluster Synchronization Services (CSS)

Manages the cluster configuration by controlling which nodes are members of the cluster and by notifying members when a node joins or leaves the cluster. If you are using third-party clusterware, then the css process interfaces with your clusterware to manage node
membership information.

Processes connected to this service are init.cssd, ocssd both run as root user and ocssd.bin run as oracle user. Failure of this process results in cluster restart.(ocssd)

Cluster Ready Services (CRS)

The primary program for managing high availability operations within a cluster. Anything that the crs process manages is known as a cluster resource which could be a database, an instance, a service, a Listener, a virtual IP (VIP) address, an application process, and so on. The crs process manages cluster resources based on the resource’s configuration information that is stored in the OCR. This includes start, stop, monitor and failover operations. The crs process generates events when a resource status changes. When you have installed RAC, crs monitors the Oracle instance, Listener, and so on, and automatically restarts these components when a failure occurs. By default, the crs process makes five attempts to restart a resource and then does not make further restart attempts if the resource does not restart.

Process connected with this service is crsd.bin and runs as root user. This process restarts automatically upon failure.

Event Management (EVM)

A background process that publishes events that crs creates.

Proceses connected with this service are evmd runs as root while evmd.bin and evmlogger runs as oracle user. This process also starts the racgevt process to manage FAN server callouts.

Oracle Notification Service (ONS)

A publish and subscribe service for communicating Fast Application Notification (FAN) events.

Ons process is connected with this service. Runs as oralce user.


Extends clusterware to support Oracle-specific requirements and complex resources. Runs server callout scripts when FAN events occur.

Process Monitor Daemon (OPROCD)

This process is locked in memory to monitor the cluster and provide I/O fencing. OPROCD performs its check, stops running, and if the wake up is beyond the expected time, then OPROCD resets the processor and reboots the node. An OPROCD failure results in the Oracle
Clusterware restarting the node. OPROCD uses the hangcheck timer on Linux platforms.

oprocd processes connected with this service and runs as root user.

  • The Real Application Clusters Software Components

To ensure that each RAC database instance obtains the block that it needs to satisfy a query or transaction, 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 System Global Area for a RAC instance.

These RAC-specific processes and the GRD collaborate to enable Cache Fusion.

The RAC-specific processe.

LMS—Global Cache Service Process

LMD—Global Enqueue Service Daemon

LMON—Global Enqueue Service Monitor

LCK0Instance Enqueue Process

The Oracle Clusterware Voting Disk and Oracle Cluster Registry

Voting Disk

Manages cluster membership by way of a health check and arbitrates cluster ownership among the instances in case of network failures. RAC uses the voting disk to determine which instances are members of a cluster. The voting disk must reside on shared disk. For high availability, Oracle recommends that you have multiple voting disks. The Oracle Clusterware enables multiple voting disks but you must have an odd number of voting disks, such as three, five, and so on. If you define a single voting disk, then you should use external
mirroring to provide redundancy.

Oracle Cluster Registry (OCR)

Maintains cluster configuration information as well as configuration information about any cluster database within the cluster. The OCR also manages information about processes that the Oracle Clusterware controls. The OCR stores configuration information in a series of key-value pairs within a directory tree structure. The OCR must reside on shared disk that is accessible by all of the nodes in your cluster. The Oracle Clusterware can multiplex the OCR and Oracle recommends that you use this feature to ensure cluster high availability. You can replace a failed OCR online, and you can update the OCR through supported APIs such as Enterprise Manager, the Server Control Utility (SRVCTL), or the Database Configuration Assistant (DBCA).