Booting a Ramdisk or an ISO

Ironic supports booting a user provided ramdisk or an ISO image (starting with the Victoria release) instead of deploying a node. Most commonly this is performed when an instance is booted via PXE, iPXE or Virtual Media, with the only local storage contents being those in memory. It is supported by pxe, ipxe, redfish-virtual-media and ilo-virtual-media boot interfaces.

Configuration

Ramdisk/ISO boot requires using the ramdisk deploy interface. It is enabled by default starting with the Zed release cycle. On an earlier release, it must be enabled explicitly:

[DEFAULT]
...
enabled_deploy_interfaces = direct,ramdisk
...

Once enabled and the conductor(s) have been restarted, the interface can be set upon creation of a new node:

baremetal node create --driver <driver> \
    --deploy-interface ramdisk \
    --boot-interface ipxe

or update an existing node:

baremetal node set <NODE> --deploy-interface ramdisk

You can also use it with redfish virtual media instead of iPXE.

Creating a ramdisk

A ramdisk can be created using the ironic-ramdisk-base element from ironic-python-agent-builder, e.g. with Debian:

export ELEMENTS_PATH=/opt/stack/ironic-python-agent-builder/dib
disk-image-create -o /output/ramdisk \
    debian-minimal ironic-ramdisk-base openssh-server dhcp-all-interfaces

You should consider using the following elements:

The resulting files (/output/ramdisk.kernel and /output/ramdisk.initramfs in this case) can then be used when Booting a ramdisk.

Booting a ramdisk

Pass the kernel and ramdisk as normally, also providing the ramdisk as an image source, for example,

baremetal node set <NODE> \
    --instance-info kernel=http://path/to/ramdisk.kernel \
    --instance-info ramdisk=http://path/to/ramdisk.initramfs
baremetal node deploy <NODE>

Booting an ISO

The ramdisk deploy interface can also be used to boot an ISO image. For example,

baremetal node set <NODE> \
    --instance-info boot_iso=http://path/to/boot.iso
baremetal node deploy <NODE>

Note

While this interface example utilizes a HTTP URL, as with all fields referencing file artifacts in the instance_info field, a user is able to request a file path URL, or an HTTPS URL, or as a Glance Image Service object UUID.

Warning

This feature, when utilized with the ipxe boot_interface, will only allow a kernel and ramdisk to be booted from the supplied ISO file. Any additional contents, such as additional ramdisk contents or installer package files will be unavailable after the boot of the Operating System. Operators wishing to leverage this functionality for actions such as OS installation should explore use of the standard ramdisk deploy_interface along with the instance_info/kernel_append_params setting to pass arbitrary settings such as a mirror URL for the initial ramdisk to load data from. This is a limitation of iPXE and the overall boot process of the operating system where memory allocated by iPXE is released.

When choosing your boot ISO, your ISO image will need to be sufficient to boot the hardware under normal conditions. For example, if the ISO is only compatible with BIOS booting, then a host in UEFI mode will not boot. This is not a limitation of Ironic, but an architectural limitation.

By default the Bare Metal service will cache the ISO locally and serve from its HTTP server. If you want to avoid that, set the following:

baremetal node set <NODE> \
    --instance-info ramdisk_image_download_source=http

ISO images are also cached across deployments, similarly to how it is done for normal instance images. The URL together with the last modified response header are used to determine if an image needs updating.

Limitations

The intended use case is for advanced scientific and ephemeral workloads where the step of writing an image to the local storage is not required or desired. As such, this interface does come with several caveats:

  • Configuration drives are not supported with network boot, only with Redfish virtual media.

  • Disk image contents are not written to the bare metal node.

  • Users and Operators who intend to leverage this interface should expect to leverage a metadata service, custom ramdisk images, or the instance_info/ramdisk_kernel_arguments parameter to add options to the kernel boot command line.

  • When using PXE/iPXE boot, bare metal nodes must continue to have network access to PXE and iPXE network resources. This is contrary to most tenant networking enabled configurations where this access is restricted to the provisioning and cleaning networks

  • As with all deployment interfaces, automatic cleaning of the node will still occur with the contents of any local storage being wiped between deployments.

Common options

Disable persistent boot device for ramdisk iso boot

For iso boot, Ironic sets the boot target to continuously boot from the iso attached over virtual media. This behaviour may not always be desired e.g. if the vmedia is installing to hard drive and then rebooting. In order to instead set the virtual media to be one time boot Ironic provides the force_persistent_boot_device flag in the node’s driver_info. Which can be set to Never:

$ openstack baremetal node set --driver-info force_persistent_boot_device='Never' <node>