PostgreSQL 10 High Performance - Third Edition

Write cache sizing

On the write side of things, Linux handles writes to the disk using a daemon named pdflush. It will spawn a number of pdflush processes (between two and eight) to keep up with the amount of outstanding I/O. pdflush is not very aggressive about writes, under the theory that if you wait longer to write things out, you will optimize total throughput. When you're writing a large data set, both write combining and being able to sort writes across more data will lower average seeking.

The main driver for when things in the write cache are aggressively written out to disk are two tuneable kernel parameters, as follows:

• /proc/sys/vm/dirty_background_ratio: Maximum percentage of active RAM that can be filled with dirty pages before ...