ceph/doc/rbd/qemu-rbd.rst

==============
 QEMU and RBD
==============

Ceph integrates with the QEMU virtual machine. For details on QEMU, see 
`QEMU Open Source Processor Emulator`_. For QEMU documentation, see
`QEMU Manual`_. 

.. important:: To use Ceph block devices with QEMU, you must have a running Ceph cluster.
   
Installing QEMU on Ubuntu 12.04 Precise
=======================================

QEMU packages are incorporated into the Ubuntu 12.04 precise distribution. To 
install QEMU on precise, execute the following:: 

	sudo apt-get install qemu

Installing QEMU on Earlier Versions of Ubuntu
=============================================

For Ubuntu distributions 11.10 Oneiric and earlier, you must install 
the 0.15 version of QEMU or later. To build QEMU from source, use the
following procedure::

	cd {your-development-directory}
	git clone git://git.qemu.org/qemu.git
	cd qemu
	./configure --enable-rbd
	make; make install

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 rbd rbd:{pool-name}/{image-name} {size}

For example::

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

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 -f rbd rbd:{pool-name}/{image-name} {size}

For example::

	qemu-img resize -f rbd rbd:data/foo 10G


Retrieving Image Information 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 -f rbd rbd:{pool-name}/{image-name}

For example::

	qemu-img info -f rbd 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 rbd 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


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>


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/