Driver BDM Data Structures

In addition to the API BDM data format there are also several internal data structures within Nova that map out how block devices are attached to instances. This document aims to outline the two general data structures and two additional specific data structures used by the libvirt virt driver.


This document is based on an email to the openstack-dev mailing list by Matthew Booth below provided as a primer for developers working on virt drivers and interacting with these data structures.


References to local disks in the following document refer to any disk directly managed by nova compute. If nova is configured to use RBD or NFS for instance disks then these disks won’t actually be local, but they are still managed locally and referred to as local disks. As opposed to RBD volumes provided by Cinder that are not considered local.

Generic BDM data structures


The ‘top level’ data structure is the BlockDeviceMapping (BDM) object. It is a NovaObject, persisted in the DB. Current code creates a BDM object for every disk associated with an instance, whether it is a volume or not.

The BDM object describes properties of each disk as specified by the user. It is initially from a user request, for more details on the format of these requests please see the Block Device Mapping in Nova document.

The Compute API transforms and consolidates all BDMs to ensure that all disks, explicit or implicit, have a BDM, and then persists them. Look in nova.objects.block_device for all BDM fields, but in essence they contain information like (source_type=’image’, destination_type=’local’, image_id=’<image uuid’>), or equivalents describing ephemeral disks, swap disks or volumes, and some associated data.


BDM objects are typically stored in variables called bdm with lists in bdms, although this is obviously not guaranteed (and unfortunately not always true: bdm in libvirt.block_device is usually a DriverBlockDevice object). This is a useful reading aid (except when it’s proactively confounding), as there is also something else typically called block_device_mapping which is not a BlockDeviceMapping object.


Changed in version 24.0.0: (Xena)

The legacy block_device_info format is no longer supported.

Drivers do not directly use BDM objects. Instead, they are transformed into a different driver-specific representation. This representation is normally called block_device_info, and is generated by virt.driver.get_block_device_info(). Its output is based on data in BDMs. block_device_info is a dict containing:


Hypervisor’s notion of the root device’s name


An image backed disk if used


A list of all ephemeral disks


A list of all cinder volumes


A swap disk, or None if there is no swap disk


The disks were previously represented in one of two ways, depending on the specific driver in use. A legacy plain dict format or the currently used DriverBlockDevice format discussed below. Support for the legacy format was removed in Xena.

Disks are represented by subclasses of nova.block_device.DriverBlockDevice. These subclasses retain a reference to the underlying BDM object. This means that by manipulating the DriverBlockDevice object, the driver is able to persist data to the BDM object in the DB.


Common usage is to pull block_device_mapping out of this dict into a variable called block_device_mapping. This is not a BlockDeviceMapping object, or a list of them.


If block_device_info was passed to the driver by compute manager, it was probably generated by _get_instance_block_device_info(). By default, this function filters out all cinder volumes from block_device_mapping which don’t currently have connection_info. In other contexts this filtering will not have happened, and block_device_mapping will contain all volumes.

libvirt driver specific BDM data structures


The virt driver API defines a method get_instance_disk_info, which returns a JSON blob. The compute manager calls this and passes the data over RPC between calls without ever looking at it. This is driver-specific opaque data. It is also only used by the libvirt driver, despite being part of the API for all drivers. Other drivers do not return any data. The most interesting aspect of instance_disk_info is that it is generated from the libvirt XML, not from nova’s state.


instance_disk_info is often named disk_info in code, which is unfortunate as this clashes with the normal naming of the next structure. Occasionally the two are used in the same block of code.


RBD disks (including non-volume disks) and cinder volumes are not included in instance_disk_info.

instance_disk_info is a list of dicts for some of an instance’s disks. Each dict contains the following:


libvirt’s notion of the disk’s type


libvirt’s notion of the disk’s path


The disk’s virtual size in bytes (the size the guest OS sees)


libvirt’s notion of the backing file path


The file size of path, in bytes.


As-yet-unallocated disk size, in bytes.



As opposed to instance_disk_info, which is frequently called disk_info.

This data structure is actually described pretty well in the comment block at the top of nova.virt.libvirt.blockinfo. It is internal to the libvirt driver. It contains:


The default bus used by disks


The default bus used by cdrom drives


Defined below

mapping is a dict which maps disk names to a dict describing how that disk should be passed to libvirt. This mapping contains every disk connected to the instance, both local and volumes.

First, a note on disk naming. Local disk names used by the libvirt driver are well defined. They are:


The root disk


The flavor-defined ephemeral disk


Where X is a zero-based index for BDM defined ephemeral disks


The swap disk


The config disk

These names are hardcoded, reliable, and used in lots of places.

In disk_info, volumes are keyed by device name, eg ‘vda’, ‘vdb’. Different buses will be named differently, approximately according to legacy Linux device naming.

Additionally, disk_info will contain a mapping for ‘root’, which is the root disk. This will duplicate one of the other entries, either ‘disk’ or a volume mapping.

Each dict within the mapping dict contains the following 3 required fields of bus, dev and type with two optional fields of format and boot_index:


The guest bus type (‘ide’, ‘virtio’, ‘scsi’, etc)


The device name ‘vda’, ‘hdc’, ‘sdf’, ‘xvde’ etc


Type of device eg ‘disk’, ‘cdrom’, ‘floppy’


Which format to apply to the device if applicable


Number designating the boot order of the device


BlockDeviceMapping and DriverBlockDevice store boot index zero-based. However, libvirt’s boot index is 1-based, so the value stored here is 1-based.


Add a section for the per disk file within instance directory when using the libvirt driver.