PostgreSQL 10 High Performance - Third Edition

Database block life cycle

The normal sequence by which a block makes its way through shared memory and either onto (or back onto) disk works like this:

1. A page to hold the database block needed is requested for allocation in the shared buffer cache. If that block already exists in there, no disk read is needed. Otherwise, ask the OS to read the page into memory. In either case, at this point, the page in memory will have its usage count increased, because presumably it's being requested so that it can be used for something. The pgstat_bgwriter.buffers_alloc statistics will be incremented to count this allocation.
2. Changes are made to the block. In the last example, an INSERT created a new row in a new block. You might also UPDATE or DELETE ...