Basics of Oracle RAC

Real Application Clusters:

Oracle Real Application Clusters (RAC) is a database clustering solution that allows more than one instance to mount and open an Oracle database. RAC can only be used on special clustered systems with shared disk storage and a private network interconnect.

Oracle RAC Architecture Diagram:

This is basic architecture of Oracle RAC, We will discuss about new features/additions in 11g in each sections.

RAC components:

• Shared Disk System
• Oracle clusterware
• Cluster interconnects
• Oracle Kernel Components

Shared Disk System

Below are the three major type of shared storage which are using in RAC

Raw volumes: A raw logical volume is an area of physical and logical disk space that is under the direct control of an application such as database or partition rather than under the direct control of the operating system or a file system. This as the only available option in 9i.

Cluster File system: This option is not widely used and here the cluster file system such as Oracle Cluster file system (OCFS) for MS Windows and Linux holding the all datafiles of RAC database

Automatic Storage Management (ASM): Oracle recommended storage option which is optimized for cluster file system for Oracle database files introduced in Oracle 10g.

Oracle Clusterware

Clusterware is the mandatory piece of software which is required to run Oracle RAC option,it provides the basic clustering support at OS level and enables oracle software to run in cluster mode.

Clusterware enables nodes to communicate each other and forms the cluster that make the nodes work as a single logical server.

Clusterware is managed by cluster ready services (CRS) using the Oracle Cluster registry (OCR) and voting disk

Cluster Ready Services (CRS)

Oracle clusterware uses CRS for interaction between the OS and the Database.CRS have five components – namely Process Monitor daemon (OPROCd: obsolete in 11gR2),CRS daemon(CRSd),Oralce Custer Synchronization Service daemon(OCSSd),Event Volume Manager daemon(EVMd) and the Oracle Notification Service (ONS)

CRS is installed and run from ORA_CRS_HOME in 10g and GRID_HOME in 11g

Starting and Stopping CRS

crsctl stop crs      # Stops Oracle clusterware

crsctl start crs     # Starts Oracle clusterware

crsctl enable crs    # Enables Oracle clusterware

crsctl disable crs   # Disables Oracle clusterware

Note: These commands should be used in root user (Superuser)

Voting Disk

A voting disk is a shared diks that will be accessed by all the member of the nodes in the cluster.It is used as   central referece for all the nodes and keeps the heartbeat information between the nodes.If any of the node is unable to ping the voding disk,cluster immediatly recogonize the comminucation failure and envicts the node from the cluster.

Cluster interconnect

It is the communication path used by the cluster for the synchronization of resources and it is also used in some cases for transfer of data from one instance to another.Typically, the interconnect is a network connections that is dedicated to the server nodes of a cluster (thus is sometimes referred as private interconnect)

Single Client Access Name (SCAN)-11gR2

SCAN is a single network name that resolves in three different IPs registered in DNS or GNS.

Prior the 11g R2 if we add or remove a node from cluster it was required to changes the connection setting in client . By configuring the connection using SCAN name this problem is eliminated.

Oracle Kernel Components 

Set of additional background process in each instance is known as oracle kernel components in RAC environment.Since buffer and shared pool became global in RAC , special handling is required to manage the resources  to avoid conflicts and corruption.Additional background process (for RAC) and single instance background process works together and achieved this.

New features in Oracle 10g/11g RAC:

Oracle 10g RAC

• CRS was renamed as Clusterware
• asmcmd introduced
• CLUVFY introduced
• OCR and Voting disks can be mirrored

Oracle 11g R2 RAC

• Oracle 11g RAC parallel upgrades - Oracle 11g have rolling upgrade features whereby RAC database can be upgraded without any downtime.
• Hot patching - Zero downtime patch application.
• We can store everything on the ASM. We can store OCR & voting files also on the ASM.
• Single Client Access Name (SCAN) - eliminates the need to change tns entry when nodes are added to or removed from the Cluster. RAC instances register to SCAN listeners as remote listeners. SCAN is fully qualified name. Oracle recommends assigning 3 addresses to SCAN, which create three SCAN listeners.
• Grid Naming Service (GNS) is a new service introduced in Oracle RAC 11g R2. With GNS, Oracle Clusterware (CRS) can manage Dynamic Host Configuration Protocol (DHCP) and DNS services for the dynamic node registration and configuration.