Config drives are special drives that are attached to an instance
when it boots. The instance can mount this drive and read files from it
to get information that is normally available through the metadata service
</admin/metadata-service>.

There are many use cases for the config drive. One such use case is
to pass a networking configuration when you do not use DHCP to assign IP
addresses to instances. For example, you might pass the IP address
configuration for the instance through the config drive, which the
instance can mount and access before you configure the network settings
for the instance. Another common reason to use config drives is load. If
running something like the OpenStack puppet providers in your instances,
they can hit the metadata servers
</admin/metadata-service> every fifteen minutes,
simultaneously for every instance you have. They are just checking in,
and building facts, but it’s not insignificant load. With a config
drive, that becomes a local (cached) disk read. Finally, using a config
drive means you’re not dependent on the metadata service being up,
reachable, or performing well to do things like reboot your instance
that runs cloud-init at the
beginning.

Any modern guest operating system that is capable of mounting an ISO
9660 or VFAT file system can use the config drive.

Requirements and guidelines

To use the config drive, you must follow the following requirements
for the compute host and image.

Compute host requirements

The following virt drivers support the config drive: libvirt, Hyper-V
and VMware. The Bare Metal service also supports the config drive.

• To use config drives with libvirt or VMware, you must first install
  the genisoimage
  package on each compute host. Use the mkisofs_cmd config option to set the
  path where you install the genisoimage program. If genisoimage is in the
  same path as the nova-compute service, you do not need to set this
  flag.
• To use config drives with Hyper-V, you must set the mkisofs_cmd
  config option to the full path to an mkisofs.exe installation. Additionally, you must
  set the hyperv.qemu_img_cmd config option to
  the full path to an qemu-img command installation.
• To use config drives with the Bare Metal service, you do not need to
  prepare anything.

Image requirements

An image built with a recent version of the cloud-init
package can automatically access metadata passed through the config
drive. The cloud-init package version 0.7.1 works with Ubuntu, Fedora
based images (such as Red Hat Enterprise Linux) and openSUSE based
images (such as SUSE Linux Enterprise Server). If an image does not have
the cloud-init package installed, you must customize the image to run a
script that mounts the config drive on boot, reads the data from the
drive, and takes appropriate action such as adding the public key to an
account. For more details about how data is organized on the config
drive, refer to the user guide <metadata-config-drive>.

Configuration

The nova-compute
service accepts the following config drive-related options:

• api.config_drive_skip_versions
• force_config_drive
• config_drive_format

If using the HyperV compute driver, the following additional options
are supported:

• hyperv.config_drive_cdrom

For example, to ensure nova always provides a config drive to
instances but versions 2018-08-27 (Rocky) and
2017-02-22 (Ocata) are skipped, add the following to nova.conf:

[DEFAULT]
force_config_drive = True

[api]
config_drive_skip_versions = 2018-08-27 2017-02-22