Oracle Clusterware Initialization

Oracle Clusterware Initialization

During the installation of Oracle Clusterware, the init.ohasd startup script is copied to /etc/init.d . The wrapper script is responsible for setting up environment variables and then starting the Oracle Clusterware daemons and processes.

The Oracle High Availability Services daemon (ohasd) is responsible for starting in proper order, monitoring, and restarting other local Oracle daemons including the crds daemon, which manages clusterwide resources. When init starts ohasd on Clusterware startup, ohasd starts orarootagent,cssdagent, and oraagent. Some of the high availability daemons will be running under the root user with real-time priority, and others will be running under the Clusterware owner with user-mode priorities after they are started. When a command is used to stop Oracle Clusterware, the daemons will be stopped, but the ohasd process will remain running.

When a cluster node boots, or Clusterware is started on a running clusterware node, the init process starts ohasd. The ohasd process then initiates the startup of the processes in the lower, or Oracle High Availability (OHASD) stack. 

• The cssdagent process is started, which in turn, starts cssd. The cssd process discovers the voting disk either in ASM or on shared storage, and then joins the cluster. The cssdagent process monitors the cluster and provides I/O fencing. This service formerly was provided by Oracle Process Monitor Daemon (oprocd). A cssdagent failure may result in Oracle Clusterware restarting the node. 

• The orarootagent is started. This process is a specialized oraagent process that helps crsd start and manage resources owned by root, such as the network and the grid virtual IP address.
  
  
• The oraagent process is started. It is responsible for starting processes that do not need to be run as root. 
  The oraagent process extends clusterware to support Oracle-specific requirements and complex resources. This process runs server callout scripts when FAN events occur. This process was known as RACG in Oracle Clusterware 11g Release 1 (11.1). 
• The cssdmonitor is started and is responsible for monitoring the cssd daemon.