Configuration Using Metadata

Boot configuration user data can be passed to a virtual machine during startup.

In this section:

For example, an EMS may be used in cloud environments for VM configuration, but the VM may require some bootstrap information to successfully communicate with the EMS.

StarlingX OpenStack provides three mechanisms to accomplish this:

User Data

This is a mechanism for passing a local file to an instance when it is launched. This method is typically used to pass a shell script or configuration file.

To send user data when calling nova boot, use the --user-data /path/to/filename option, or use the Heat service and set the user\_data property and user\_data\_format to RAW.

On initialization, the VM queries the metadata service through either the EC2 compatibility API. For example:

$ curl http://169.254.169.254/2012-08-10/user_data

or the OpenStack metadata API. For example:

$ curl http://169.254.169.254/openstack/2009-04-04/user_data

In either case, text is returned to the VM and can be used for bootstrap configuration.

Access to the metadata server at 169.254.169.254 is provided by a virtual router attached to the project network on which the access request is made. Virtual routers are automatically configured as proxies to the metadata service.

cloud-init

This is an open-source package available from https://cloudinit.readthedocs.org/en/latest/ that supports a variety of guests. It expects a particular file format for user data, retrieves the user data from the metadata server, and takes action based on the contents of the data.

Two commonly used input formats are:

shell scripts

You can configure an instance at boot time by passing a shell script as user data. The script file must begin with

#!

for cloud-init to recognize it as a shell script.

Cloud config files

A configuration format based on YAML that you can use to configure a large number of options on a system. For example, to set the hostname:

#cloud-config
hostname: mynode
fqdn: mynode.example.com
manage_etc_hosts: true

See https://cloudinit.readthedocs.org/en/latest for a complete list of capabilities.

Config drive

StarlingX OpenStack can be configured to use a special-purpose configuration drive (abbreviated config drive) to store metadata (including injected files). Metadata is written to the drive, which is attached to the instance when it boots. The instance can retrieve information normally available through the metadata service by reading from the mounted drive.

The config drive can be enabled by using the --config-drive=true option with nova boot.

The following example enables the config drive and passes user data, injecting two files and two key/value metadata pairs. These can be read from the config drive.

$ openstack server create --config-drive true --image my-image-name --flavor 1 --key-name mykey --user-data ./my-user-data.txt --property role=webservers --property essential=false MYINSTANCE

From within the instance, the config drive volume is labeled config-2. You can mount the config drive as the /dev/disk/by-label/config-2 device if your guest OS supports accessing disks by label. For example:

# mkdir -p /mnt/config
# mount /dev/disk/by-label/config-2 /mnt/config

The contents of the config drive depend on the options passed to nova boot. The contents of the config drive for the example above are:

ec2/2009-04-04/meta-data.json
ec2/2009-04-04/user-data
ec2/latest/meta-data.json
ec2/latest/user-data
openstack/2012-08-10/meta_data.json
openstack/2012-08-10/user_data
openstack/content
openstack/content/0000
openstack/content/0001
openstack/latest/meta_data.json
openstack/latest/user_data

For file format details and full details on config-drive, refer to the public OpenStack documentation.

Caution

If a VM uses config-drive for user data or file injection, VM evacuations due to a compute node failure and VM live migrations to another compute node will cause the config drive to be rebuilt on the new compute node and metadata to be populated, but user data and injected files are not populated in the evacuated or live-migrated config drive of the VM.

For a VM using config-file with file injection, it is recommended to copy the injected files to the root disk of the VM on initial boot, and to set a flag to prevent the use of injected files on subsequent boots.

File injection is disabled when using a Ceph backend.