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Describe running a vm from RBD, cache options, and discard support. Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
120 lines
3.9 KiB
ReStructuredText
120 lines
3.9 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 RBD 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 RBD Images with QEMU
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-----------------------------
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You can create an RBD image from QEMU. You must specify ``rbd``,
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the pool name, and the name of the image you wish to create. You must also
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specify 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 RBD Images with QEMU
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-----------------------------
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You can resize an RBD 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 RBD Image Information with QEMU
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------------------------------------------
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You can retrieve RBD 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 RBD image as a block device on
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the host. Instead, QEMU can access an RBD 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 RBD.
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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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Using `RBD caching`_ can significantly improve performance.
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Since QEMU 1.2, QEMU's cache options control RBD 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 RBD 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
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.. _RBD caching: ../../config-cluster/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, RBD supports the discard
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operation. This means that a guest can send TRIM requests to let RBD
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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 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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