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# Oracle RAC High Availability Services Processes and crsctl Commands
## Understanding the Startup Process of RAC Nodes
When working with Oracle RAC clusters, it's essential to understand the sequence in which critical services are started and stopped. This sequence ensures the smooth operation and stability of the cluster.
### Step-by-Step Startup of RAC1
1. **Starting Oracle High Availability Services (OHASD)**
   - Use the following command to start the OHASD process on RAC1:
     ```bash
     ./crsctl start crs -excl
     ```
   - The output will indicate the successful startup of OHASD and subsequent services.
2. **Subsequent Service Starts**
   - The starting order of services on RAC1 typically follows this sequence:
     - `ora.gipcd`
     - `ora.mdnsd`
     - `ora.gpnpd`
     - `ora.cssdmonitor`
     - `ora.cssd`
     - `ora.asm`
     - `ora.crsd`
   - Each service may have its own start and stop attempts, ensuring that the cluster is built correctly.
---
## Shutdown Process of RAC Nodes
Similar to the startup procedure, the shutdown process follows a specific order to ensure that all resources are released without conflicts.
### Step-by-Step Shutdown of RAC1
1. **Stopping the Oracle High Availability Services (OHASD)**
   - Use the following command to stop the OHASD process on RAC1:
     ```bash
     ./crsctl stop crs -f
     ```
   - The output will display the stopping of each service in reverse order.
2. **Subsequent Service Stops**
   - The services are stopped in the following reverse order:
     - `ora.crsd`
     - `ora.asm`
     - `ora.cssd`
     - `ora.cssdmonitor`
     - `ora.gpnpd`
     - `ora.mdnsd`
     - `ora.gipcd`
   - Each service must be properly stopped before proceeding to the next, ensuring that no resources are left hanging.
---
## Working with Exclusive Mode (-excl)
When starting services in exclusive mode (-excl), ensure that no other nodes are attempting to use these resources simultaneously.
### Understanding Exclusive Mode
- By using `-excl` with crsctl start crs, you instruct Oracle to use exclusive access modes for critical services.
- This prevents other nodes from starting these services simultaneously, thereby avoiding resource conflicts.
### Observing Resource Contention
When rac2 attempts to start services in exclusive mode while rac1 is already running, it may result in issues such as:
- `CRS-4402`: Attempting to start a CSS daemon on a node where it already exists.
- `CRS-2674`: Failure to start the CSS daemon due to active instances on other nodes.
This highlights the importance of proper clustering and mutual exclusion in RAC environments.
---
## Differences Between crsctl start crs and crsctl start cluster
### crsctl start crs
- This command starts both the OHASD and the cluster service (CSSD) processes.
- It's typically used when you want to start all related High Availability services at once.
### crsctl start cluster
- This command only starts the CRSC (CSSD) and its associated services.
- It is intended for managed services only after OHASD has been started.
### Key Difference
- `crsctl start crs` manages OHASD, while `crsctl start cluster` excludes it. Always start OHASD before using `crsctl start cluster` for cluster services.
---
## Summary
Understanding the sequence of service starts and stops, along with the proper use of exclusive modes, is crucial for maintaining the integrity of your Oracle RAC cluster. Always ensure that services are stopped and started in the correct order, and use `crsctl` commands appropriately depending on your operational requirements.