Write-Ahead Log / Recovery

recovery_prefetch

• What it does: Enables or disables the prefetching of data blocks during recovery by looking ahead in the WAL to identify blocks that will be needed soon.
• Why it matters: This parameter significantly accelerates recovery times by anticipating data block needs and loading them into memory before they're actually required. During recovery, PostgreSQL can read ahead in the WAL stream to identify which data blocks will be needed for upcoming operations and prefetch them, reducing I/O wait times and parallelizing disk access. This is particularly beneficial for large databases where recovery time is critical for availability.
• Ideal value & Best Practice: Default on is recommended for most environments. The performance improvement is usually significant with minimal overhead. Only disable if you experience issues with prefetching behavior or for troubleshooting recovery problems. Ensure adequate I/O bandwidth is available to handle the additional prefetch requests.

wal_decode_buffer_size

• What it does: Sets the size of the buffer used for reading ahead in the WAL during recovery to identify blocks for prefetching.
• Why it matters: This parameter controls how far ahead the recovery process can look in the WAL stream to identify blocks that need prefetching. A larger buffer allows for more aggressive prefetching and better anticipation of future block needs, which can significantly improve recovery performance. However, larger buffers consume more memory and may not provide additional benefits beyond a certain point depending on your WAL pattern and I/O capabilities.
• Ideal value & Best Practice: Default 512KB is conservative. For faster recovery, increase to 2-4MB for most systems. For very large databases with high WAL volume, consider 8-16MB. Monitor recovery performance and adjust based on your specific workload characteristics. The optimal value depends on your WAL generation rate and I/O subsystem performance.

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