Oracle DBA

1. ohasd Startup:
   • The Oracle High Availability Services Daemon (ohasd) is initiated by the operating system’s initialization process, typically as part of the system boot.
   • ohasd is responsible for managing and coordinating various clusterware components.
2. OLR Access:
   • Upon startup, ohasd accesses the OLR (Oracle Local Registry). The OLR is a local configuration repository stored on each cluster member. It contains critical configuration information for cluster nodes and services.
3. gpnpd and cssd Startup:
   • After initializing itself using information from the OLR, ohasd proceeds to start the gpnpd (Grid Plug and Play Daemon) and cssd (Cluster Synchronization Services Daemon) processes.
4. gpnp Profile:
   • gpnpd reads the gpnp profile, which contains cluster configuration details such as the ASM disk string and ASM SPFILE location. This information is essential for understanding the cluster’s structure.
5. Voting Disk Discovery:
   • cssd (Cluster Synchronization Services Daemon) plays a crucial role in cluster membership and arbitration.
   • It scans all ASM disk headers to locate the voting disk. The voting disk is a shared disk that is essential for cluster membership and ensuring the cluster’s stability.
   • Once the voting disk is identified, cssd reads it using specialized tools like kfed. This information helps establish and maintain cluster membership.
6. ASM SPFILE Retrieval:
   • To start the ASM instance, ohasd needs the ASM SPFILE, which stores configuration settings for the ASM instance.
   • ohasd reads the ASM SPFILE’s location from the gpnp profile. However, it doesn’t need to physically open the disk to access the SPFILE.
   • The necessary information about the ASM SPFILE is stored in the ASM disk header. ohasd reads the header of the ASM disk containing the SPFILE (as determined from the gpnp profile) and retrieves the SPFILE contents using tools like kfed.
   • With the ASM SPFILE information in hand, ohasd starts the ASM instance.
7. OCR Access and CRSD Startup:
   • The Oracle Cluster Registry (OCR), which stores cluster configuration data and resource definitions, is typically stored within an ASM disk group.
   • Once the ASM instance is up and running, ohasd can access the OCR.
   • After OCR access is available, ohasd proceeds to start the Cluster Ready Services Daemon (CRSD). The CRSD is responsible for managing cluster resources and services.

So, in summary, the files and processes involved in the Oracle Clusterware startup sequence are as follows:

1. OLR (Oracle Local Registry) – Accessed by ohasd.
2. GPNP profile (Grid Plug and Play Profile) – Read by gpnpd.
3. Voting Disk – Used by cssd for cluster membership and arbitration.
4. ASM SPFILE – Read by ohasd from ASM disk header information.
5. OCR (Oracle Cluster Registry) – Managed by CRSD and stored in an ASM disk group.

Oracle Clusterware startup sequence involves a carefully orchestrated process of initializing key components, accessing configuration information, discovering critical cluster resources like the voting disk, retrieving the ASM SPFILE from disk headers, and finally, starting the ASM instance and CRSD. This sequence ensures the proper functioning and high availability of the Oracle cluster.