User data is a special key in the metadata service that holds a file that cloud-aware applications in the guest instance can access. For example the cloudinit system is a Ubuntu open source package that handles early initialization of a cloud instance and that makes use of user data. You can place user data in a local file and pass it through the --user-data <user-data-file> parameter at instance creation: $ nova boot --image ubuntu-cloudimage --flavor 1 --user-data mydata.file