Umair Shahid explains what dirty pages are in PostgreSQL:
PostgreSQL stores data in fixed‑size blocks (pages), normally 8 KB. When a client updates or inserts data, PostgreSQL does not immediately write those changes to disk. Instead, it loads the affected page into shared memory (shared buffers), makes the modification there, and marks the page as dirty. A “dirty page” means the version of that page in memory is newer than the on‑disk copy.
Before returning control to the client, PostgreSQL records the change in the Write‑Ahead Log (WAL), ensuring durability even if the database crashes. However, the actual table file isn’t updated until a checkpoint or background writer flushes the dirty page to disk.
The concept is the same in SQL Server. Read on to see how PostgreSQL manages dirty pages and some of the issues you might run into with them.