Config drives

Note

This section provides deployment information about the config drive feature. For end-user information about the config drive feature and instance metadata in general, refer to the user guide.

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.

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 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.

Configuration

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

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

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

Note

The img_config_drive image metadata property can be used to force enable the config drive. In addition, users can explicitly request a config drive when booting instances. For more information, refer to the user guide.