Cache hit ratio below 99% indicates undersized shared_buffers or high disk I/O

Cache Hit Ratio falls below 99% indicating that shared_buffers may be undersized for current workload, causing excessive physical disk I/O as PostgreSQL cannot find requested pages in memory.

Run the cache efficiency diagnosis query to measure hits vs disk reads. If cache_hit_ratio_percent is below 99% and disk I/O is high, increase shared_buffers parameter (start with 25% of total RAM, rarely exceed 40% or 16-32GB). Use ALTER SYSTEM SET shared_buffers = '16GB'; then restart PostgreSQL.