Don't let this picture scare you too much. You aren't responsible for managing all of these processes, that is the Clusterware's job!
Short summary of the startup sequence: INIT spawns init.ohasd (with respawn) which in turn starts the OHASD process (Oracle High Availability Services Daemon). This daemon spawns 4 processes.
Level 1: OHASD Spawns:
- cssdagent - Agent responsible for spawning CSSD.
- orarootagent - Agent responsible for managing all root owned ohasd resources.
- oraagent - Agent responsible for managing all oracle owned ohasd resources.
- cssdmonitor - Monitors CSSD and node health (along wth the cssdagent).
Level 2: OHASD rootagent spawns:
- CRSD - Primary daemon responsible for managing cluster resources.
- CTSSD - Cluster Time Synchronization Services Daemon
- Diskmon
- ACFS (ASM Cluster File System) Drivers
Level 2: OHASD oraagent spawns:
- MDNSD - Used for DNS lookup
- GIPCD - Used for inter-process and inter-node communication
- GPNPD - Grid Plug & Play Profile Daemon
- EVMD - Event Monitor Daemon
- ASM - Resource for monitoring ASM instances
Level 3: CRSD spawns:
- orarootagent - Agent responsible for managing all root owned crsd resources.
- oraagent - Agent responsible for managing all oracle owned crsd resources.
Level 4: CRSD rootagent spawns:
- Network resource - To monitor the public network
- SCAN VIP(s) - Single Client Access Name Virtual IPs
- Node VIPs - One per node
- ACFS Registery - For mounting ASM Cluster File System
- GNS VIP (optional) - VIP for GNS
Level 4: CRSD oraagent spawns:
- ASM Resouce - ASM Instance(s) resource
- Diskgroup - Used for managing/monitoring ASM diskgroups.
- DB Resource - Used for monitoring and managing the DB and instances
- SCAN Listener - Listener for single client access name, listening on SCAN VIP
- Listener - Node listener listening on the Node VIP
- Services - Used for monitoring and managing services
- ONS - Oracle Notification Service
- eONS - Enhanced Oracle Notification Service
- GSD - For 9i backward compatibility
- GNS (optional) - Grid Naming Service - Performs name resolution
This image shows the various levels more clearly:
