Re: [PERFORM] A Better External Sort?

> Now I've asked for the quickest path to detailed
> understanding of the pg IO subsystem.  The goal being to get
> more up to speed on its coding details.  Certainly not to
> annoy you or anyone else.

Basically pg does random 8k (compile time blocksize) reads/writes only.
Bitmap and sequential scans read 8k blocks in order.
Only WAL does n x 8k writes with one system call.

pg relys on the OS readahead (== larger block IO) to do efficient IO.
Basically the pg scan performance should match a dd if=file of=/dev/null
bs=8k,
unless CPU bound.

Andreas