Store MySQL's data directory in /var/ramfs

[edmorley]

If /var/ramfs exists, the postgresql cookbook copies the existing Postgresql data directory to it, and then updates data_directory to use the new location:

travis-cookbooks/cookbooks/travis_postgresql/templates/ubuntu/initd_postgresql.erb

Lines 55 to 64 in 5c6d733

	if [ -d /var/ramfs ]; then
	# Copy initial databases to RAMFS
	for data_dir in /var/lib/postgresql/*; do
	v=$(basename $data_dir)
	if [ -d $(dirname /var/ramfs/postgresql) -a ! -d /var/ramfs/postgresql/$v ]; then
	mkdir -p /var/ramfs/postgresql/$v
	cp -rp $data_dir /var/ramfs/postgresql/
	fi
	done
	fi

travis-cookbooks/cookbooks/travis_postgresql/templates/ubuntu/9.6/postgresql.conf.erb

Lines 41 to 45 in 5c6d733

	<% if ::File.directory?('/var/ramfs') -%>
	data_directory = '/var/ramfs/postgresql/9.6/main' # use data in another directory
	<% else -%>
	data_directory = '/var/lib/postgresql/9.6/main' # use data in another directory
	<% end -%>

It would be great to do something similar for MySQL, since doing so manually (using the steps here) made our unit tests run twice as fast.

This would presumably mean:

1. Copying the contents of /var/lib/mysql/ to /var/ramfs/mysql/ in a similar manner to what initd_postgresql.erb does.
2. Overriding the MySQL datadir option via an extra /etc/mysql/conf.d/ config file.