========================
 QEMU and Block Devices
========================

.. index:: Ceph Block Device; QEMU KVM

The most frequent Ceph Block Device use case involves providing block device
images to virtual machines. For example, a user may create  a "golden" image
with an OS and any relevant software in an ideal configuration. Then the user
takes a snapshot of the image. Finally the user clones the snapshot (potentially
many times). See `Snapshots`_ for details. The ability to make copy-on-write
clones of a snapshot means that Ceph can provision block device images to
virtual machines quickly, because the client doesn't have to download the entire
image each time it spins up a new virtual machine.


.. ditaa::

            +---------------------------------------------------+
            |                       QEMU                        |
            +---------------------------------------------------+
            |                      librbd                       |
            +---------------------------------------------------+
            |                     librados                      |
            +------------------------+-+------------------------+
            |          OSDs          | |        Monitors        |
            +------------------------+ +------------------------+


Ceph Block Devices attach to QEMU virtual machines. For details on
QEMU, see  `QEMU Open Source Processor Emulator`_. For QEMU documentation, see
`QEMU Manual`_. For installation details, see `Installation`_.

.. important:: To use Ceph Block Devices with QEMU, you must have access to a 
   running Ceph cluster.


Usage
=====

The QEMU command line expects you to specify the Ceph pool and image name. You
may also specify a snapshot. 

QEMU will assume that Ceph configuration resides in the default
location (e.g., ``/etc/ceph/$cluster.conf``) and that you are executing
commands as the default ``client.admin`` user unless you expressly specify
another Ceph configuration file path or another user. When specifying a user,
QEMU uses the ``ID`` rather than the full ``TYPE:ID``. See `User Management -
User`_ for details. Do not prepend the client type (i.e., ``client.``) to the
beginning of the user  ``ID``, or you will receive an authentication error. You
should have the key for the ``admin`` user or the key of another user you
specify with the ``:id={user}`` option in a keyring file stored in default path
(i.e., ``/etc/ceph`` or the local directory with appropriate file ownership and
permissions. Usage takes the following form::

	qemu-img {command} [options] rbd:{pool-name}/{image-name}[@snapshot-name][:option1=value1][:option2=value2...]

For example, specifying the ``id`` and ``conf`` options might look like the following:: 

	qemu-img {command} [options] rbd:glance-pool/maipo:id=glance:conf=/etc/ceph/ceph.conf

.. tip:: Configuration values containing ``:``, ``@``, or ``=`` can be escaped with a
         leading ``\`` character.


Creating Images with QEMU
=========================

You can create a block device image from QEMU. You must specify ``rbd``,  the
pool name, and the name of the image you wish to create. You must also specify
the size of the image. ::

	qemu-img create -f raw rbd:{pool-name}/{image-name} {size}

For example::

	qemu-img create -f raw rbd:data/foo 10G

.. important:: The ``raw`` data format is really the only sensible
   ``format`` option to use with RBD. Technically, you could use other
   QEMU-supported formats (such as ``qcow2`` or ``vmdk``), but doing
   so would add additional overhead, and would also render the volume
   unsafe for virtual machine live migration when caching (see below)
   is enabled.


Resizing Images with QEMU
=========================

You can resize a block device image from QEMU. You must specify ``rbd``, 
the pool name, and the name of the image you wish to resize. You must also
specify the size of the image. ::

	qemu-img resize rbd:{pool-name}/{image-name} {size}

For example::

	qemu-img resize rbd:data/foo 10G


Retrieving Image Info with QEMU
===============================

You can retrieve block device image information from QEMU. You must 
specify ``rbd``, the pool name, and the name of the image. ::

	qemu-img info rbd:{pool-name}/{image-name}

For example::

	qemu-img info rbd:data/foo


Running QEMU with RBD
=====================

QEMU can pass a block device from the host on to a guest, but since
QEMU 0.15, there's no need to map an image as a block device on
the host. Instead, QEMU attaches an image as a virtual block
device directly via ``librbd``. This strategy increases performance
by avoiding context switches and taking advantage of `RBD caching`_.

You can use ``qemu-img`` to convert existing virtual machine images to Ceph
block device images. For example, if you have a qcow2 image, you could run::

    qemu-img convert -f qcow2 -O raw debian_squeeze.qcow2 rbd:data/squeeze

To run a virtual machine booting from that image, you could run::

    qemu -m 1024 -drive format=raw,file=rbd:data/squeeze

`RBD caching`_ can significantly improve performance.
Since QEMU 1.2, QEMU's cache options control ``librbd`` caching::

    qemu -m 1024 -drive format=rbd,file=rbd:data/squeeze,cache=writeback

If you have an older version of QEMU, you can set the ``librbd`` cache
configuration (like any Ceph configuration option) as part of the
'file' parameter::

    qemu -m 1024 -drive format=raw,file=rbd:data/squeeze:rbd_cache=true,cache=writeback

.. important:: If you set rbd_cache=true, you must set cache=writeback
   or risk data loss. Without cache=writeback, QEMU will not send
   flush requests to librbd. If QEMU exits uncleanly in this
   configuration, file systems on top of rbd can be corrupted.

.. _RBD caching: ../rbd-config-ref/#rbd-cache-config-settings


.. index:: Ceph Block Device; discard trim and libvirt

Enabling Discard/TRIM
=====================

Since Ceph version 0.46 and QEMU version 1.1, Ceph Block Devices support the
discard operation. This means that a guest can send TRIM requests to let a Ceph
block device reclaim unused space. This can be enabled in the guest by mounting
``ext4`` or ``XFS`` with the ``discard`` option.

For this to be available to the guest, it must be explicitly enabled
for the block device. To do this, you must specify a
``discard_granularity`` associated with the drive::

    qemu -m 1024 -drive format=raw,file=rbd:data/squeeze,id=drive1,if=none \
         -device driver=ide-hd,drive=drive1,discard_granularity=512

Note that this uses the IDE driver. The virtio driver supports discard since Linux kernel version 5.0.

If using libvirt, edit your libvirt domain's configuration file using ``virsh
edit`` to include the ``xmlns:qemu`` value. Then, add a ``qemu:commandline``
block as a child of that domain. The following example shows how to set two
devices with ``qemu id=`` to different ``discard_granularity`` values.

.. code-block:: xml

	<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
		<qemu:commandline>
			<qemu:arg value='-set'/>
			<qemu:arg value='block.scsi0-0-0.discard_granularity=4096'/>
			<qemu:arg value='-set'/>
			<qemu:arg value='block.scsi0-0-1.discard_granularity=65536'/>
		</qemu:commandline>
	</domain>


.. index:: Ceph Block Device; cache options

QEMU Cache Options
==================

QEMU's cache options correspond to the following Ceph `RBD Cache`_ settings.

Writeback::

   rbd_cache = true

Writethrough::

   rbd_cache = true
   rbd_cache_max_dirty = 0

None:: 

   rbd_cache = false

QEMU's cache settings override Ceph's cache settings (including settings that
are explicitly set in the Ceph configuration file).

.. note:: Prior to QEMU v2.4.0, if you explicitly set `RBD Cache`_ settings
   in the Ceph configuration file, your Ceph settings override the QEMU cache
   settings.

.. _QEMU Open Source Processor Emulator: http://wiki.qemu.org/Main_Page
.. _QEMU Manual: http://wiki.qemu.org/Manual
.. _RBD Cache: ../rbd-config-ref/
.. _Snapshots: ../rbd-snapshot/
.. _Installation: ../../install
.. _User Management - User: ../../rados/operations/user-management#user
