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156 lines
4.9 KiB
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156 lines
4.9 KiB
ReStructuredText
==============
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QEMU and RBD
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==============
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Ceph integrates with the QEMU virtual machine. For details on QEMU, see
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`QEMU Open Source Processor Emulator`_. For QEMU documentation, see
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`QEMU Manual`_.
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.. important:: To use Ceph block devices with QEMU, you must have a running Ceph cluster.
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Installing QEMU on Ubuntu 12.04 Precise
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=======================================
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QEMU packages are incorporated into the Ubuntu 12.04 precise distribution. To
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install QEMU on precise, execute the following::
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sudo apt-get install qemu
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Installing QEMU on Earlier Versions of Ubuntu
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=============================================
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For Ubuntu distributions 11.10 Oneiric and earlier, you must install
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the 0.15 version of QEMU or later. To build QEMU from source, use the
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following procedure::
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cd {your-development-directory}
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git clone git://git.qemu.org/qemu.git
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cd qemu
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./configure --enable-rbd
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make; make install
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Creating Images with QEMU
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=========================
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You can create a block device image from QEMU. You must specify ``rbd``, the
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pool name, and the name of the image you wish to create. You must also specify
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the size of the image. ::
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qemu-img create -f rbd rbd:{pool-name}/{image-name} {size}
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For example::
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qemu-img create -f rbd rbd:data/foo 10G
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Resizing Images with QEMU
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=========================
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You can resize a block device image from QEMU. You must specify ``rbd``,
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the pool name, and the name of the image you wish to resize. You must also
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specify the size of the image. ::
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qemu-img resize -f rbd rbd:{pool-name}/{image-name} {size}
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For example::
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qemu-img resize -f rbd rbd:data/foo 10G
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Retrieving Image Information with QEMU
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======================================
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You can retrieve block device image information from QEMU. You must
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specify ``rbd``, the pool name, and the name of the image. ::
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qemu-img info -f rbd rbd:{pool-name}/{image-name}
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For example::
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qemu-img info -f rbd rbd:data/foo
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Running QEMU with RBD
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=====================
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QEMU can pass a block device from the host on to a guest, but since
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QEMU 0.15, there's no need to map an image as a block device on
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the host. Instead, QEMU can access an image as a virtual block
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device directly via ``librbd``. This performs better because it avoids
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an additional context switch, and can take advantage of `RBD caching`_.
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You can use ``qemu-img`` to convert existing virtual machine images to Ceph
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block device images. For example, if you have a qcow2 image, you could run::
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qemu-img convert -f qcow2 -O rbd debian_squeeze.qcow2 rbd:data/squeeze
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To run a virtual machine booting from that image, you could run::
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qemu -m 1024 -drive format=raw,file=rbd:data/squeeze
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`RBD caching`_ can significantly improve performance.
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Since QEMU 1.2, QEMU's cache options control ``librbd`` caching::
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qemu -m 1024 -drive format=rbd,file=rbd:data/squeeze,cache=writeback
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If you have an older version of QEMU, you can set the ``librbd`` cache
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configuration (like any Ceph configuration option) as part of the
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'file' parameter::
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qemu -m 1024 -drive format=raw,file=rbd:data/squeeze:rbd_cache=true,cache=writeback
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.. important:: If you set rbd_cache=true, you must set cache=writeback
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or risk data loss. Without cache=writeback, QEMU will not send
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flush requests to librbd. If QEMU exits uncleanly in this
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configuration, filesystems on top of rbd can be corrupted.
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.. _RBD caching: ../rbd-config-ref/#rbd-cache-config-settings
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Enabling Discard/TRIM
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=====================
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Since Ceph version 0.46 and QEMU version 1.1, Ceph block devices support the
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discard operation. This means that a guest can send TRIM requests to let a Ceph
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block device reclaim unused space. This can be enabled in the guest by mounting
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ext4 or XFS with the ``discard`` option.
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For this to be available to the guest, it must be explicitly enabled
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for the block device. To do this, you must specify a
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``discard_granularity`` associated with the drive::
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qemu -m 1024 -drive format=raw,file=rbd:data/squeeze,id=drive1,if=none \
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-device driver=ide-hd,drive=drive1,discard_granularity=512
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Note that this uses the IDE driver. The virtio driver does not
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support discard.
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QEMU Cache Options
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==================
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QEMU's cache options correspond to the following Ceph `RBD Cache`_ settings.
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Writeback::
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rbd_cache = true
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Writethrough::
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rbd_cache = true
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rbd_cache_max_dirty = 0
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None::
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rbd_cache = false
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QEMU's cache settings override Ceph's default settings (i.e., settings that are
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not explicitly set in the Ceph configuration file). If you explicitly set `RBD
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Cache`_ settings in your Ceph configuration file, your Ceph settings override
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the QEMU cache settings. If you set cache settings on the QEMU command line, the
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QEMU command line settings override the Ceph configuration file settings.
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.. _QEMU Open Source Processor Emulator: http://wiki.qemu.org/Main_Page
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.. _QEMU Manual: http://wiki.qemu.org/Manual
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.. _RBD Cache: ../rbd-config-ref/
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