Moving the MySQL Tmpdir on Ubuntu

One thing I frequently forget when changing the default directories for various services in Ubuntu is that the AppArmor config for those services also needs to be updated. Case in point, the other day I needed to change MySQL's tmpdir to a different disk, and struggled for a while with "Can't create/write to file 'xyz' (Errcode: 13)" errors until I remembered AppArmor. These were the steps I ended up taking:

Create the new tmpdir

mkdir /mnt/foo/tmp && sudo chown mysql:mysql /mnt/foo/tmp

Change /etc/mysql/my.cnf to use the new tmpdir

tmpdir = /mnt/foo/tmp

Add new tmpdir entries to /etc/apparmor.d/local/usr.sbin.mysqld

/mnt/foo/tmp/ r,
/mnt/foo/tmp/** rw,

Reload AppArmor

sudo service apparmor reload

Restart MySQL

sudo service mysql restart

While I was troubleshooting, I found a nice, in-depth blog entry by Jeremy Smyth explaining how to debug issues with AppArmor and MySQL.