WARNING: could not flush dirty data: Function not implemented

Ubuntu 18.04 as Windows bash

Distributor ID: Ubuntu
Description:    Ubuntu 18.04.1 LTS
Release:        18.04
Codename:       bionic


PG 10.5.1

postgres@ravi-lenovo:~$ psql -d postgres
psql (10.5 (Ubuntu 10.5-1.pgdg16.04+1))

A CREATE DATABASE statement spewed out 

WARNING:  could not flush dirty data: Function not implemented

many times, but the db was created.

What exactly is this?

A simple search tells that this is an old problem and my Windows has necessary WSL and other subsystem for this error
tonot show up.