Oracle RAC Architecture

A peek into RAC Architecture

Let's begin with a brief overview of RAC architecture.

• A cluster is a set of 2 or more machines (nodes) that share or coordinate resources to perform the same task.
• A RAC database is 2 or more instances running on a set of clustered nodes, with all instances accessing a shared set of database files.
• Depending on the O/S platform, a RAC database may be deployed on a cluster that uses vendor clusterware plus Oracle's own clusterware (Cluster Ready Services), or on a cluster that solely uses Oracle's own clusterware.

Thus, every RAC sits on a cluster that is running Cluster Ready Services. srvctl is the primary tool DBAs use to configure CRS for their RAC database and processes.

Cluster Ready Services and the OCR

Cluster Ready Services, or CRS, is a new feature for 10g RAC. Essentially, it is Oracle's own clusterware. On most platforms, Oracle supports vendor clusterware; in these cases, CRS interoperates with the vendor clusterware, providing high availability support and service and workload management. On Linux and Windows clusters, CRS serves as the sole clusterware. In all cases, CRS provides a standard cluster interface that is consistent across all
platforms.
CRS consists of four processes (crsd, occsd, evmd, and evmlogger) and two disks: the Oracle Cluster Registry (OCR), and the voting disk.
CRS manages the following resources:

• The ASM instances on each node
• Databases
• The instances on each node
• Oracle Services on each node
• The cluster nodes themselves, including the following processes, or "nodeapps":
  
  • VIP
  • GSD
  • The listener
  • The ONS daemon

CRS stores information about these resources in the OCR. If the information in the OCR for one of these resources becomes damaged or inconsistent, then CRS is no longer able to manage that resource. Fortunately, the OCR automatically backs itself up regularly and frequently.

Interacting with CRS and the OCR: srvctl

srvctl is the tool Oracle recommends that DBAs use to interact with CRS and the cluster registry. Oracle does provide several tools to interface with the cluster registry and CRS more directly, at a lower level, but these tools are deliberately undocumented and intended only for use by Oracle Support. srvctl, in contrast, is well documented and easy to use. Using other tools to modify the OCR or manage CRS without the assistance of Oracle Support runs the risk of damaging the OCR.

Using srvctl

Even if you are experienced with 9i srvctl, it's worth taking a look at this section; 9i and 10g srvctl commands are slightly different.
srvctl must be run from the $ORACLE_HOME of the RAC you are administering. The basic format of a srvctl command is
srvctl <command> <target> [options]
where command is one of
enable|disable|start|stop|relocate|status|add|remove|modify|getenv|setenv|unsetenv|config
and the target, or object, can be a database, instance, service, ASM instance, or the nodeapps.