How Postgres Writes Data: WAL, Shared Buffers, and the Illusion of Speed

Short summary

PostgreSQL achieves sub-millisecond write responses through three architectural layers: in-memory Shared Buffers cache modified pages, the append-only Write-Ahead Log ensures durability on disk via sequential I/O, and a background Checkpointer periodically flushes dirty pages to permanent storage. The database uses OS process isolation instead of threads for fault tolerance, and connection pooling decouples millions of users from expensive database processes.

• •Shared Buffers cache table data in RAM; dirty pages are logged to WAL before confirming success.
• •WAL uses fast sequential I/O (appending) instead of slow random I/O (scattered table updates).
• •Process isolation and connection pooling enable safe, scalable concurrent access.

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