Oracle Clusterware 10g, formerly known as Cluster Ready Services (CRS) is software that when installed on servers running the same operating system, enables the servers to be bound together to operate and function as a single server or cluster. This infrastructure simplifies the requirement for an Oracle Real Application Clusters (RAC) database by providing cluster software that is tightly integrated with the Oracle Database.
The Oracle Clusterware requires two critical clusterware components: a voting disk to record node membership information and the Oracle Cluster Registry (OCR) to record cluster configuration information:
Voting Disk
The voting disk is a shared partition that Oracle Clusterware uses to verify cluster node membership and status. Oracle Clusterware uses the voting disk to determine which instances are members of a cluster by way of a health check and arbitrates cluster ownership among the instances in case of network failures. The primary function of the voting disk is to manage node membership and prevent what is known as Split Brain Syndrome in which two or more instances attempt to control the RAC database. This can occur in cases where there is a break in communication between nodes through the interconnect.
The voting disk must reside on a shared disk(s) that is accessible by all of the nodes in the cluster. For high availability, Oracle recommends that you have multiple voting disks. Oracle Clusterware can be configured to maintain multiple voting disks (multiplexing) but you must have an odd number of voting disks, such as three, five, and so on. Oracle Clusterware supports a maximum of 32 voting disks. If you define a single voting disk, then you should use external mirroring to provide redundancy.
A node must be able to access more than half of the voting disks at any time. For example, if you have five voting disks configured, then a node must be able to access at least three of the voting disks at any time. If a node cannot access the minimum required number of voting disks it is evicted, or removed, from the cluster. After the cause of the failure has been corrected and access to the voting disks has been restored, you can instruct Oracle Clusterware to recover the failed node and restore it to the cluster.
Oracle Cluster Registry (OCR)
Maintains cluster configuration information as well as configuration information about any cluster database within the cluster. OCR is the repository of configuration information for the cluster that manages information about like the cluster node list and instance-to-node mapping information. This configuration information is used by many of the processes that make up the CRS as well as other cluster-aware applications which use this repository to share information amoung them. Some of the main components included in the OCR are:
- Node membership information
- Database instance, node, and other mapping information
- ASM (if configured)
- Application resource profiles such as VIP addresses, services, etc.
- Service characteristics
- Information about processes that Oracle Clusterware controls
- Information about any third-party applications controlled by CRS (10g R2 and later)
The OCR stores configuration information in a series of key-value pairs within a directory tree structure. To view the contents of the OCR in a human-readable format, run the ocrdump command. This will dump the contents of the OCR into an ASCII text file in the current directory named OCRDUMPFILE.
The OCR must reside on a shared disk(s) that is accessible by all of the nodes in the cluster. Oracle Clusterware 10g Release 2 allows you to multiplex the OCR and Oracle recommends that you use this feature to ensure cluster high availability. Oracle Clusterware allows for a maximum of two OCR locations; one is the primary and the second is an OCR mirror. If you define a single OCR, then you should use external mirroring to provide redundancy. 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).
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