Boot From Volume


The Bare Metal service supports booting from a Cinder iSCSI volume as of the Pike release. This guide will primarily deal with this use case, but will be updated as more paths for booting from a volume, such as FCoE, are introduced.

The boot from volume is supported on both legacy BIOS and UEFI (iPXE binary for EFI booting) boot mode. We need to perform with suitable images which will be created by diskimage-builder tool.

How this works - From Ironic’s point of view

In essence, ironic sets the stage for the process, by providing the required information to the boot interface to facilitate the configuration of the the node OR the iPXE boot templates such that the node CAN be booted.


In this example, the boot interface does the heavy lifting. For drivers the irmc and ilo hardware types with hardware type specific boot interfaces, they are able to signal via an out of band mechanism to the baremetal node’s BMC that the integrated iSCSI initiators are to connect to the supplied volume target information.

In most hardware this would be the network cards of the machine.

In the case of the ipxe boot interface, templates are created on disk which point to the iscsi target information that was either submitted as part of the volume target, or when integrated with Nova, what was requested as the baremetal’s boot from volume disk upon requesting the instance.

In terms of network access, both interface methods require connectivity to the iscsi target. In the vendor driver specific path, additional network configuration options may be available to allow separation of standard network traffic and instance network traffic. In the iPXE case, this is not possible as the OS userspace re-configures the iSCSI connection after detection inside the OS ramdisk boot.

An iPXE user may be able to leverage multiple VIFs, one specifically set to be set with pxe_enabled to handle the initial instance boot and back-end storage traffic where as external facing network traffic occurs on a different interface. This is a common pattern in iSCSI based deployments in the physical realm.


Currently booting from a volume requires:

  • Bare Metal service version 9.0.0

  • Bare Metal API microversion 1.33 or later

  • A driver that utilizes the PXE boot mechanism. Currently booting from a volume is supported by the reference drivers that utilize PXE boot mechanisms when iPXE is enabled.

  • iPXE is an explicit requirement, as it provides the mechanism that attaches and initiates booting from an iSCSI volume.

  • Metadata services need to be configured and available for the instance images to obtain configuration such as keys. Configuration drives are not supported due to minimum disk extension sizes.

Conductor Configuration

In ironic.conf, you can specify a list of enabled storage interfaces. Check [DEFAULT]enabled_storage_interfaces in your ironic.conf to ensure that your desired interface is enabled. For example, to enable the cinder and noop storage interfaces:

enabled_storage_interfaces = cinder,noop

If you want to specify a default storage interface rather than setting the storage interface on a per node basis, set [DEFAULT]default_storage_interface in ironic.conf. The default_storage_interface will be used for any node that doesn’t have a storage interface defined.

Node Configuration

Storage Interface

You will need to specify what storage interface the node will use to handle storage operations. For example, to set the storage interface to cinder on an existing node:

baremetal node set --storage-interface cinder $NODE_UUID

A default storage interface can be specified in ironic.conf. See the Conductor Configuration section for details.

iSCSI Configuration

In order for a bare metal node to boot from an iSCSI volume, the iscsi_boot capability for the node must be set to True. For example, if you want to update an existing node to boot from volume:

baremetal node set --property capabilities=iscsi_boot:True $NODE_UUID

You will also need to create a volume connector for the node, so the storage interface will know how to communicate with the node for storage operation. In the case of iSCSI, you will need to provide an iSCSI Qualifying Name (IQN) that is unique to your SAN. For example, to create a volume connector for iSCSI:

baremetal volume connector create \
         --node $NODE_UUID --type iqn --connector-id$NODE_UUID

Image Creation

We use disk-image-create in diskimage-builder tool to create images for boot from volume feature. Some required elements for this mechanism for corresponding boot modes are as following:

  • Legacy BIOS boot mode: iscsi-boot element.

  • UEFI boot mode: iscsi-boot and block-device-efi elements.

An example below:

export IMAGE_NAME=<image_name>
export DIB_CLOUD_INIT_DATASOURCES="ConfigDrive, OpenStack"
disk-image-create centos7 vm cloud-init-datasources dhcp-all-interfaces iscsi-boot dracut-regenerate block-device-efi -o $IMAGE_NAME


  • For CentOS images, we must add dependent element named dracut-regenerate during image creation. Otherwise, the image creation will fail with an error.

  • For Ubuntu images, we only support iscsi-boot element without dracut-regenerate element during image creation.

Advanced Topics

Use without the Compute Service

As discussed in other sections, the Bare Metal service has a concept of a connector that is used to represent an interface that is intended to be utilized to attach the remote volume.

In addition to the connectors, we have a concept of a target that can be defined via the API. While a user of this feature through the Compute service would automatically have a new target record created for them, it is not explicitly required, and can be performed manually.

A target record can be created using a command similar to the example below:

baremetal volume target create \
          --node $NODE_UUID --type iscsi --boot-index 0 --volume $VOLUME_UUID


A boot-index value of 0 represents the boot volume for a node. As the boot-index is per-node in sequential order, only one boot volume is permitted for each node.

Use Without Cinder

In the Rocky release, an external storage interface is available that can be utilized without a Block Storage Service installation.

Under normal circumstances the cinder storage interface interacts with the Block Storage Service to orchestrate and manage attachment and detachment of volumes from the underlying block service system.

The external storage interface contains the logic to allow the Bare Metal service to determine if the Bare Metal node has been requested with a remote storage volume for booting. This is in contrast to the default noop storage interface which does not contain logic to determine if the node should or could boot from a remote volume.

It must be noted that minimal configuration or value validation occurs with the external storage interface. The cinder storage interface contains more extensive validation, that is likely un-necessary in a external scenario.

Setting the external storage interface:

baremetal node set --storage-interface external $NODE_UUID

Setting a volume:

baremetal volume target create --node $NODE_UUID \
    --type iscsi --boot-index 0 --volume-id $VOLUME_UUID \
    --property target_iqn="" \
    --property target_lun="0" \
    --property target_portal="" \
    --property auth_method="CHAP" \
    --property auth_username="ABC" \
    --property auth_password="XYZ" \

Ensure that no image_source is defined:

baremetal node unset \
    --instance-info image_source $NODE_UUID

Deploy the node:

baremetal node deploy $NODE_UUID

Upon deploy, the boot interface for the baremetal node will attempt to either create iPXE configuration OR set boot parameters out-of-band via the management controller. Such action is boot interface specific and may not support all forms of volume target configuration. As of the Rocky release, the bare metal service does not support writing an Operating System image to a remote boot from volume target, so that also must be ensured by the user in advance.

Records of volume targets are removed upon the node being undeployed, and as such are not persistent across deployments.

Cinder Multi-attach

Volume multi-attach is a function that is commonly performed in computing clusters where dedicated storage subsystems are utilized. For some time now, the Block Storage service has supported the concept of multi-attach. However, the Compute service, as of the Pike release, does not yet have support to leverage multi-attach. Concurrently, multi-attach requires the backend volume driver running as part of the Block Storage service to contain support for multi-attach volumes.

When support for storage interfaces was added to the Bare Metal service, specifically for the cinder storage interface, the concept of volume multi-attach was accounted for, however has not been fully tested, and is unlikely to be fully tested until there is Compute service integration as well as volume driver support.

The data model for storage of volume targets in the Bare Metal service has no constraints on the same target volume from being utilized. When interacting with the Block Storage service, the Bare Metal service will prevent the use of volumes that are being reported as in-use if they do not explicitly support multi-attach.