doc/rbd/libvirt.rst - main - githubmirror/ceph

doc/rbd/libvirt.rst
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Ville Ojamo
doc: Use validated links with ref instead of external links
08 дек 2025, 09:55
08 дек 2025, 09:55a4108d9
=================================
 Using libvirt with Ceph RBD
=================================

.. index:: Ceph Block Device; livirt

The ``libvirt`` library creates a virtual machine abstraction layer between
hypervisor interfaces and the software applications that use them. With
``libvirt``, developers and system administrators can focus on a common
management framework, common API, and common shell interface (i.e., ``virsh``)
to many different hypervisors, including:

- QEMU/KVM
- XEN
- LXC
- VirtualBox
- etc.

Ceph block devices support QEMU/KVM. You can use Ceph block devices with
software that interfaces with ``libvirt``. The following stack diagram
illustrates how ``libvirt`` and QEMU use Ceph block devices via ``librbd``.


.. ditaa::

            +---------------------------------------------------+
            |                     libvirt                       |
            +------------------------+--------------------------+
                                     |
                                     | configures
                                     v
            +---------------------------------------------------+
            |                       QEMU                        |
            +---------------------------------------------------+
            |                      librbd                       |
            +---------------------------------------------------+
            |                     librados                      |
            +------------------------+-+------------------------+
            |          OSDs          | |        Monitors        |
            +------------------------+ +------------------------+


The most common ``libvirt`` use case involves providing Ceph block devices to
cloud solutions like OpenStack, OpenNebula or CloudStack. The cloud solution uses
``libvirt`` to  interact with QEMU/KVM, and QEMU/KVM interacts with Ceph block
devices via  ``librbd``. See `Block Devices and OpenStack`_,
`Block Devices and OpenNebula`_ and `Block Devices and CloudStack`_ for details.
See :ref:`install-overview` for installation details.

You can also use Ceph block devices with ``libvirt``, ``virsh`` and the
``libvirt`` API. See `libvirt Virtualization API`_ for details.


To create VMs that use Ceph block devices, use the procedures in the following
sections. In the exemplary embodiment, we have used ``libvirt-pool`` for the pool
name, ``client.libvirt`` for the user name, and ``new-libvirt-image`` for  the
image name. You may use any value you like, but ensure you replace those values
when executing commands in the subsequent procedures.


Configuring Ceph
================

To configure Ceph for use with ``libvirt``, perform the following steps:

#. :ref:`Create a pool <createpool>`. The following example uses the
   pool name ``libvirt-pool``.::

	ceph osd pool create libvirt-pool

   Verify the pool exists. ::

	ceph osd lspools

#. Use the ``rbd`` tool to initialize the pool for use by RBD::

        rbd pool init <pool-name>

#. :ref:`Create a Ceph User <rados_ops_adding_a_user>` (or use ``client.admin`` for version 0.9.7 and
   earlier). The following example uses the Ceph user name ``client.libvirt``
   and references ``libvirt-pool``. ::

	ceph auth get-or-create client.libvirt mon 'profile rbd' osd 'profile rbd pool=libvirt-pool'

   Verify the name exists. ::

	ceph auth ls

   **NOTE**: ``libvirt`` will access Ceph using the ID ``libvirt``,
   not the Ceph name ``client.libvirt``. See :ref:`User Management - User <rados-ops-user>` and
   `User Management - CLI`_ for a detailed explanation of the difference
   between ID and name.

#. Use QEMU to `create an image`_ in your RBD pool.
   The following example uses the image name ``new-libvirt-image``
   and references ``libvirt-pool``. ::

	qemu-img create -f rbd rbd:libvirt-pool/new-libvirt-image 2G

   Verify the image exists. ::

	rbd -p libvirt-pool ls

   **NOTE:** You can also use `rbd create`_ to create an image, but we
   recommend ensuring that QEMU is working properly.

.. tip:: Optionally, if you wish to enable debug logs and the admin socket for
   this client, you can add the following section to ``/etc/ceph/ceph.conf``::

	[client.libvirt]
	log file = /var/log/ceph/qemu-guest-$pid.log
	admin socket = /var/run/ceph/$cluster-$type.$id.$pid.$cctid.asok

   The ``client.libvirt`` section name should match the cephx user you created
   above.
   If SELinux or AppArmor is enabled, note that this could prevent the client
   process (qemu via libvirt) from doing some operations, such as writing logs
   or operate the images or admin socket to the destination locations (``/var/
   log/ceph`` or ``/var/run/ceph``). Additionally, make sure that the libvirt
   and qemu users have appropriate access to the specified directory.


Preparing the VM Manager
========================

You may use ``libvirt`` without a VM manager, but you may find it simpler to
create your first domain with ``virt-manager``.

#. Install a virtual machine manager. See `KVM/VirtManager`_ for details. ::

	sudo apt-get install virt-manager

#. Download an OS image (if necessary).

#. Launch the virtual machine manager. ::

	sudo virt-manager



Creating a VM
=============

To create a VM with ``virt-manager``, perform the following steps:

#. Press the **Create New Virtual Machine** button.

#. Name the new virtual machine domain. In the exemplary embodiment, we
   use the name ``libvirt-virtual-machine``. You may use any name you wish,
   but ensure you replace ``libvirt-virtual-machine`` with the name you
   choose in subsequent commandline and configuration examples. ::

	libvirt-virtual-machine

#. Import the image. ::

	/path/to/image/recent-linux.img

   **NOTE:** Import a recent image. Some older images may not rescan for
   virtual devices properly.

#. Configure and start the VM.

#. You may use ``virsh list`` to verify the VM domain exists. ::

	sudo virsh list

#. Login to the VM (root/root)

#. Stop the VM before configuring it for use with Ceph.


Configuring the VM
==================

When configuring the VM for use with Ceph, it is important  to use ``virsh``
where appropriate. Additionally, ``virsh`` commands often require root
privileges  (i.e., ``sudo``) and will not return appropriate results or notify
you that root privileges are required. For a reference of ``virsh``
commands, refer to `Virsh Command Reference`_.


#. Open the configuration file with ``virsh edit``. ::

	sudo virsh edit {vm-domain-name}

   Under ``<devices>`` there should be a ``<disk>`` entry. ::

	<devices>
		<emulator>/usr/bin/kvm</emulator>
		<disk type='file' device='disk'>
			<driver name='qemu' type='raw'/>
			<source file='/path/to/image/recent-linux.img'/>
			<target dev='vda' bus='virtio'/>
			<address type='drive' controller='0' bus='0' unit='0'/>
		</disk>


   Replace ``/path/to/image/recent-linux.img`` with the path to the OS image.
   The minimum kernel for using the faster ``virtio`` bus is 2.6.25. See
   `Virtio`_ for details.

   **IMPORTANT:** Use ``sudo virsh edit`` instead of a text editor. If you edit
   the configuration file under ``/etc/libvirt/qemu`` with a text editor,
   ``libvirt`` may not recognize the change. If there is a discrepancy between
   the contents of the XML file under ``/etc/libvirt/qemu`` and the result of
   ``sudo virsh dumpxml {vm-domain-name}``, then your VM may not work
   properly.


#. Add the Ceph RBD image you created as a ``<disk>`` entry. ::

	<disk type='network' device='disk'>
		<source protocol='rbd' name='libvirt-pool/new-libvirt-image'>
			<host name='{monitor-host}' port='6789'/>
		</source>
		<target dev='vdb' bus='virtio'/>
	</disk>

   Replace ``{monitor-host}`` with the name of your host, and replace the
   pool and/or image name as necessary. You may add multiple ``<host>``
   entries for your Ceph monitors. The ``dev`` attribute is the logical
   device name that will appear under the ``/dev`` directory of your
   VM. The optional ``bus`` attribute indicates the type of disk device to
   emulate. The valid settings are driver specific (e.g., "ide", "scsi",
   "virtio", "xen", "usb" or "sata").

   See `Disks`_ for details of the ``<disk>`` element, and its child elements
   and attributes.

#. Save the file.

#. If your Ceph Storage Cluster has :ref:`rados-cephx-config-ref`
   enabled (it does by default), you must generate a secret. ::

	cat > secret.xml <<EOF
	<secret ephemeral='no' private='no'>
		<usage type='ceph'>
			<name>client.libvirt secret</name>
		</usage>
	</secret>
	EOF

#. Define the secret. ::

	sudo virsh secret-define --file secret.xml
	{uuid of secret}

#. Get the ``client.libvirt`` key and save the key string to a file. ::

	ceph auth get-key client.libvirt | sudo tee client.libvirt.key

#. Set the UUID of the secret. ::

	sudo virsh secret-set-value --secret {uuid of secret} --base64 $(cat client.libvirt.key) && rm client.libvirt.key secret.xml

   You must also set the secret manually by adding the following ``<auth>``
   entry to the ``<disk>`` element you entered earlier (replacing the
   ``uuid`` value with the result from the command line example above). ::

	sudo virsh edit {vm-domain-name}

   Then, add ``<auth></auth>`` element to the domain configuration file::

	...
	</source>
	<auth username='libvirt'>
		<secret type='ceph' uuid='{uuid of secret}'/>
	</auth>
	<target ...


   **NOTE:** The exemplary ID is ``libvirt``, not the Ceph name
   ``client.libvirt`` as generated at step 2 of `Configuring Ceph`_. Ensure
   you use the ID component of the Ceph name you generated. If for some reason
   you need to regenerate the secret, you will have to execute
   ``sudo virsh secret-undefine {uuid}`` before executing
   ``sudo virsh secret-set-value`` again.


Summary
=======

Once you have configured the VM for use with Ceph, you can start the VM.
To verify that the VM and Ceph are communicating, you may perform the
following procedures.


#. Check to see if Ceph is running::

	ceph health

#. Check to see if the VM is running. ::

	sudo virsh list

#. Check to see if the VM is communicating with Ceph. Replace
   ``{vm-domain-name}`` with the name of your VM domain::

	sudo virsh qemu-monitor-command --hmp {vm-domain-name} 'info block'

#. Check to see if the device from ``<target dev='vdb' bus='virtio'/>`` exists::

       virsh domblklist {vm-domain-name} --details

If everything looks okay, you may begin using the Ceph block device
within your VM.


.. _libvirt Virtualization API: http://www.libvirt.org
.. _Block Devices and OpenStack: ../rbd-openstack
.. _Block Devices and OpenNebula: https://docs.opennebula.io/stable/open_cluster_deployment/storage_setup/ceph_ds.html#datastore-internals
.. _Block Devices and CloudStack: ../rbd-cloudstack
.. _create an image: ../qemu-rbd#creating-images-with-qemu
.. _Virsh Command Reference: http://www.libvirt.org/virshcmdref.html
.. _KVM/VirtManager: https://help.ubuntu.com/community/KVM/VirtManager
.. _Disks: http://www.libvirt.org/formatdomain.html#elementsDisks
.. _rbd create: ../rados-rbd-cmds#creating-a-block-device-image
.. _User Management - CLI: ../../rados/operations/user-management#command-line-usage
.. _Virtio: http://www.linux-kvm.org/page/Virtio