Double buffering wastes RAM when shared_buffers exceeds 40% of total memory

Allocating more than 40% of RAM to shared_buffers causes the same data page to reside duplicated in both shared_buffers and OS Page Cache, while other hot pages are evicted to disk, generating unnecessary physical I/O.

Review shared_buffers configuration. Start with 25% of total RAM for dedicated servers. Reduce if currently above 40% to allow sufficient space for OS Page Cache. Monitor cache hit ratio to validate changes. Exceptions exist for purely analytical OLAP workloads that fit entirely in RAM.