Apache Cassandra employs several background processes that are critical to its operation and performance. Understanding these processes helps in optimizing Cassandra deployments for maximum efficiency.
Key Background Processes
1. Compaction
Compaction is one of the most resource-intensive background processes in Cassandra. It merges multiple SSTables (Sorted String Tables) into a single new one, discarding tombstones and outdated data.
Performance impacts:
- Consumes significant I/O resources during execution
- Temporarily increases disk space usage (requires space for both source and target SSTables)
- Can cause read/write latency spikes if not properly managed
Compaction strategies:
- SizeTieredCompactionStrategy (STCS): Default strategy, groups similarly-sized SSTables
- LeveledCompactionStrategy (LCS): Maintains SSTables in levels, better for read-heavy workloads
- TimeWindowCompactionStrategy (TWCS): Optimized for time-series data
2. Repair
Repair processes ensure data consistency across replicas by comparing and synchronizing data.
Performance impacts:
- Network-intensive operation
- Can cause significant load on nodes during execution
- May impact query latency during heavy repair operations
3. Garbage Collection (JVM)
As a Java application, Cassandra is subject to JVM garbage collection.
Performance impacts:
- GC pauses can cause latency spikes
- Improper GC configuration can lead to “stop-the-world” pauses
- Memory pressure affects overall performance
4. Memtable Flush
When memtables (in-memory data structures) reach their configured size limit, they’re flushed to disk as SSTables.
Performance impacts:
- I/O intensive during flush operations
- Can cause write latency spikes if not properly tuned
- Affects overall throughput during heavy write loads
5. Read Repair
Triggered during read operations when inconsistencies are detected between replicas.
Performance impacts:
- Adds overhead to read operations
- Network traffic increases with higher consistency levels
- Can improve long-term performance by maintaining data consistency
6. Streaming
Used during operations like bootstrap, decommission, and repair to transfer data between nodes.
Performance impacts:
- Network-intensive process
- Can saturate network bandwidth during large data transfers
- May impact cluster performance during topology changes
Performance Optimization Strategies
- Compaction Tuning:
- Select appropriate compaction strategy based on workload
- Configure compaction throughput limits to prevent resource starvation
- Schedule major compactions during off-peak hours
- Repair Management:
- Implement incremental repairs to reduce impact
- Schedule repairs during low-traffic periods
- Stagger repairs across the cluster
- JVM Tuning:
- Configure appropriate heap size
- Select suitable garbage collector (G1GC recommended for newer versions)
- Monitor and tune GC parameters
- Memtable Configuration:
- Adjust memtable size based on available memory
- Configure memtable_flush_writers appropriately
- Balance between memory usage and flush frequency
- Consistency Level Selection:
- Choose appropriate consistency levels to balance between performance and data integrity
- Consider LOCAL_QUORUM for most operations to reduce network traffic
By understanding and properly configuring these background processes, you can significantly improve Cassandra’s performance, stability, and resource utilization in production environments.
