ceph/doc/rbd/qemu-rbd.rst

========================
 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 (usually
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 an entire
image each time it spins up a new virtual machine.


.. ditaa::  +---------------------------------------------------+
            |                       QEMU                        |
            +---------------------------------------------------+
            |                      librbd                       |
            +---------------------------------------------------+
            |                     librados                      |
            +------------------------+-+------------------------+
            |          OSDs          | |        Monitors        |
            +------------------------+ +------------------------+


Ceph Block Devices can integrate with the QEMU virtual machine. 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.


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 can access an image as a virtual block
device directly via ``librbd``. This performs better because it avoids
an additional context switch, and can take 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, filesystems 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 does not
support discard.

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:: guess

	<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 default settings (i.e., settings that are
not explicitly set in the Ceph configuration file). If you  explicitly set `RBD
Cache`_ settings in your Ceph configuration file, your Ceph settings override
the QEMU cache settings. If you set cache settings on the QEMU command line, the
QEMU command line settings override the Ceph configuration file 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