Understanding Why OS RAM and Postgres Buffer Cache Compete

Many PostgreSQL performance issues aren’t caused by too little RAM, but by allocating memory to the wrong layer. Postgres and the OS both cache the same data independently, creating a “double buffering” problem that wastes memory and increases latency. This guide explains how shared_buffers and the OS page cache interact, why the 25% shared_buffers rule matters, how to diagnose cache pressure with pg_buffercache, and when tuning stops helping because the dataset itself has outgrown memory.