Clusterware Startup Sequence in Oracle RAC

 

Here's a step-by-step explanation of the Oracle Clusterware initialization process, detailing which process starts another process and what is read or accessed at each step:

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Step 1: init.ohasd Starts OHASD

1. What happens?

   • The init.ohasd script is invoked during server boot.
   • It is responsible for starting the Oracle High Availability Services Daemon (OHASD).

2. What does OHASD do?

• OHASD is the foundational process for Oracle Clusterware and is critical for initializing other cluster services.

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Step 2: OHASD Reads the Oracle Local Registry (OLR)

1. What happens?

   • OHASD accesses the Oracle Local Registry (OLR) file located on the local filesystem.
   • The OLR contains configuration data required for starting cluster services at the local node level.

2. Why OLR?

• OLR is essential for bootstrapping the cluster, as it contains information about the local node and processes that must start before the Cluster Registry (OCR) is accessible.

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Step 3: OHASD Starts GPNPD and CSSD

1. GPNPD (Grid Plug and Play Daemon):

   • Purpose: Manages Grid Plug and Play (GPnP) profile.
   • The GPnP profile contains critical cluster configuration information, such as:
     • ASM disk discovery strings (ASM_DISKSTRING).
     • ASM SPFILE location.
     • Cluster node membership information.
   • GPNPD provides this information to other cluster services like CSSD.

2. CSSD (Cluster Synchronization Services Daemon):

• Purpose: Establishes the cluster and maintains node membership by reading:
  • GPnP profile (to get disk discovery strings and other configuration data).
  • ASM disk headers to locate voting disks.
• How does it read voting disks?
• The KFED command is used internally to scan ASM disk headers and identify the voting disk location.
• Voting disks are used to determine node membership and ensure no split-brain scenarios occur.

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Step 4: OHASD Reads ASM Disk Headers for SPFILE

1. What happens?
• OHASD scans ASM disk headers using the discovery string provided in the GPnP profile.
• It searches for the location of the ASM SPFILE (Server Parameter File), which is required to start the ASM instance.

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Step 5: OHASD Starts ASM Instance

1. Who starts the ASM instance?
• OHASD starts the ASM instance using the SPFILE found on the ASM disk.
• The ASM instance provides access to shared storage, including:
• OCR (Oracle Cluster Registry).
• Voting Disks.
• Other ASM-managed files.

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Step 6: ASM Instance Provides Access to OCR

1. What happens?
• Once the ASM instance is up and running, it enables access to the OCR.
• The OCR contains the global cluster configuration, including information about resources and services in the cluster.

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Step 7: OHASD Starts CRSD

1. Who starts CRSD?

   • OHASD starts the Cluster Ready Services Daemon (CRSD) after the ASM instance is available and the OCR is accessible.

2. What does CRSD do?

• CRSD manages high-availability resources in the cluster, such as:
  • Database instances.
  • Listeners.
  • Virtual IPs (VIPs).
  • ASM instances.
• CRSD ensures resources are started, stopped, and failed over as needed.

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