Setting effective_io_concurrency in PostgreSQL can significantly influence database performance, especially in environments where the database is running on storage that can handle multiple concurrent IO operations efficiently, such as Solid State Drives (SSDs).

Understanding effective_io_concurrency

• Purpose: This configuration parameter is used to help the PostgreSQL planner more accurately estimate the cost of performing concurrent IO operations.
• Applicability: It's particularly relevant for systems with storage that can perform well under concurrent IO requests, like SSDs. For traditional spinning hard drives, this setting is less impactful because their ability to handle concurrent IO is limited.

How It Influences Performance

1. Cost Estimation:
   • When effective_io_concurrency is set to a value greater than 0, PostgreSQL's planner assumes that multiple IO operations can be performed in parallel.
   • This changes the cost estimation for sequential scans and bitmap heap scans. The planner may favor these scans over index scans when it expects that reading from the disk will be fast.
2. Parallel IO Operations:
   • On systems with high IO capacity (like SSDs), setting a higher effective_io_concurrency value can lead to better utilization of the IO capabilities.
   • This can result in faster query execution times, especially for IO-bound queries involving large sequential scans.
3. Balancing Act:
   • A higher value can lead to more aggressive parallelism in IO operations, but setting this value too high might lead to resource contention, especially in a busy system with many concurrent queries.
   • Conversely, setting it too low on a high-performance IO system may underutilize the IO capabilities, leading to suboptimal performance.

Best Practices

1. System Specific:
   • The optimal value depends on your specific hardware and workload. For SSDs, start with a moderate value (like 100-200) and adjust based on performance testing.
2. Monitoring and Tuning:
   • Monitor your system's performance before and after changing this setting. Pay special attention to IO-bound queries and overall system throughput.
3. Use With Other Settings:
   • effective_io_concurrency should be considered in conjunction with other settings like random_page_cost, which also influences the planner's choice between sequential and index scans.
4. Version Considerations:
   • Always check the documentation for your specific PostgreSQL version, as the behavior and impact of this setting can change between versions.
5. Test in Staging Environment:
   • As with any performance tuning, test changes in a staging environment that closely mimics your production environment to avoid unexpected issues.

In summary, effective_io_concurrency is a powerful setting in PostgreSQL for optimizing the performance of IO-bound workloads, especially on systems with high-performance IO subsystems. The key is to find the right balance for your specific environment through careful testing and monitoring.