How PostgreSQL Uses WAL and Shared Buffers to Deliver Near-Instant Write Speeds

PostgreSQL achieves fast write performance by avoiding slow random disk writes at the moment a query is executed. Instead, it first modifies data in a RAM-based cache called Shared Buffers, marking changed pages as 'dirty' until they can be persisted. To prevent data loss if the system crashes, Postgres records every transaction in a Write-Ahead Log (WAL) — an append-only file on disk — before confirming success to the application. Because appending to the WAL is sequential and far faster than random disk writes, durability is guaranteed without sacrificing speed. A background process called the Checkpointer periodically flushes dirty pages from RAM to permanent storage files, completing the full write cycle.