Oracle RAC Background Processes
Oracle RAC Background Processes – Explained in Detail
Oracle Real Application Clusters (RAC) is designed to provide high availability, scalability, and fault tolerance by allowing multiple instances to access a single database simultaneously.
Behind this powerful architecture lies a set of specialized background processes that coordinate communication, cache management, and workload distribution across nodes.
Understanding these background processes is critical for Oracle DBAs, especially when troubleshooting RAC performance or instance failures.
Why RAC Needs Special Background Processes
In a single-instance database, background processes manage memory, I/O, and recovery locally.
In Oracle RAC, however:
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Multiple instances share the same database
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Cache consistency must be maintained across nodes
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Redo and data block coordination is required
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Inter-node communication becomes essential
This is where RAC-specific background processes come into play.
Key Oracle RAC Background Processes
1. LMON (Global Enqueue Service Monitor)
Role:
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Manages the Global Enqueue Service (GES)
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Handles cluster reconfiguration during:
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Node eviction
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Instance crash
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Node join or leave
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Why it’s important:
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Ensures cluster consistency
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Detects and resolves split-brain scenarios
LMON is the backbone of RAC cluster stability.
2. LMD (Global Enqueue Service Daemon)
Role:
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Handles global lock requests
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Manages lock conversions and deadlock detection
Key function:
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Communicates lock status between instances
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Ensures that multiple instances don’t corrupt shared data
Think of LMD as the lock traffic controller in RAC.
3. LMS (Global Cache Service Process)
Role:
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Manages Global Cache Service (GCS)
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Transfers data blocks between instances via interconnect
Key responsibilities:
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Cache Fusion operations
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Block shipping (current and consistent read blocks)
Why it matters:
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Enables RAC to avoid disk I/O for block transfers
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Improves performance significantly
LMS is the heart of Cache Fusion.
4. LCK0 (Lock Process)
Role:
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Works with LMD to manage global locks
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Maintains lock structures and states
Note:
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Not heavily involved in day-to-day performance issues
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Plays a supporting role in global locking
5. DIAG (Diagnosability Process)
Role:
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Monitors cluster health
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Tracks node membership and heartbeat information
Why it’s useful:
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Helps in diagnosing:
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Node evictions
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Interconnect failures
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Cluster instability
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6. ACMS (Atomic Controlfile to Memory Service)
Role:
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Synchronizes control file metadata
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Keeps instance memory structures consistent
Ensures all RAC instances have a consistent view of the database structure.
7. GTXn (Global Transaction Process)
Role:
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Manages distributed transactions
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Used mainly in:
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XA transactions
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Parallel server environments
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Common scenario:
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When transactions span multiple instances
Difference Between Single Instance and RAC Background Processes
| Single Instance | Oracle RAC |
|---|---|
| Local cache management | Global cache coordination |
| No interconnect usage | High-speed interconnect required |
| Simple locking | Distributed locking & cache fusion |
How These Processes Work Together
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LMS handles block transfers
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LMD + LCK manage global locks
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LMON ensures cluster consistency
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DIAG monitors cluster health
All of them communicate constantly over the private interconnect, making RAC highly scalable and resilient.
DBA Perspective: Why You Should Know This
Understanding RAC background processes helps you:
✔ Analyze AWR and RAC wait events
✔ Troubleshoot GC (Global Cache) waits
✔ Handle node eviction scenarios
✔ Explain RAC internals confidently in interviews
Final Thoughts
Oracle RAC background processes are not just theoretical concepts—they are actively involved in every transaction that spans across nodes.
Mastering these processes gives you a strong foundation in RAC architecture and troubleshooting.
If you’re serious about becoming a skilled Oracle RAC DBA, this is must-know knowledge.
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