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9292cc215a
Explain why people should be using the "raw" image format for RBD
volumes created for use by QEMU: using any other format adds only
overhead, but no extra value (since RBDs are also CoW and
thin-provisioned), plus the Qcow2 storage driver is not migration safe
when caching is enabled, whereas the RBD driver is.
Also, fix a minor glitch in the example qemu-img commands ("-f rbd"
and "-O rbd" should really be "-f raw" and "-O raw").
Finally, drop the "-f" option altogether on qemu-img commands where it
makes no sense (info and resize).
Signed-off-by: Florian Haas <florian@hastexo.com>
189 lines
6.7 KiB
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189 lines
6.7 KiB
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========================
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QEMU and Block Devices
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========================
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.. index:: Ceph Block Device; QEMU KVM
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The most frequent Ceph Block Device use case involves providing block device
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images to virtual machines. For example, a user may create a "golden" image
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with an OS and any relevant software in an ideal configuration. Then, the user
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takes a snapshot of the image. Finally, the user clones the snapshot (usually
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many times). See `Snapshots`_ for details. The ability to make copy-on-write
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clones of a snapshot means that Ceph can provision block device images to
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virtual machines quickly, because the client doesn't have to download an entire
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image each time it spins up a new virtual machine.
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.. ditaa:: +---------------------------------------------------+
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| QEMU |
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+---------------------------------------------------+
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| librbd |
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+---------------------------------------------------+
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| librados |
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+------------------------+-+------------------------+
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| OSDs | | Monitors |
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+------------------------+ +------------------------+
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Ceph Block Devices can integrate with the QEMU virtual machine. For details on
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QEMU, see `QEMU Open Source Processor Emulator`_. For QEMU documentation, see
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`QEMU Manual`_. For installation details, see `Installation`_.
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.. important:: To use Ceph Block Devices with QEMU, you must have access to a
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running Ceph cluster.
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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 raw rbd:{pool-name}/{image-name} {size}
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For example::
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qemu-img create -f raw rbd:data/foo 10G
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.. important:: The ``raw`` data format is really the only sensible
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``format`` option to use with RBD. Technically, you could use other
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QEMU-supported formats (such as ``qcow2`` or ``vmdk``), but doing
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so would add additional overhead, and would also render the volume
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unsafe for virtual machine live migration when caching (see below)
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is enabled.
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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 rbd:{pool-name}/{image-name} {size}
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For example::
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qemu-img resize rbd:data/foo 10G
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Retrieving Image Info 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 rbd:{pool-name}/{image-name}
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For example::
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qemu-img info 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 raw 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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.. index:: Ceph Block Device; discard trim and libvirt
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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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If using libvirt, edit your libvirt domain's configuration file using ``virsh
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edit`` to include the ``xmlns:qemu`` value. Then, add a ``qemu:commandline``
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block as a child of that domain. The following example shows how to set two
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devices with ``qemu id=`` to different ``discard_granularity`` values.
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.. code-block:: guess
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<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
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<qemu:commandline>
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<qemu:arg value='-set'/>
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<qemu:arg value='block.scsi0-0-0.discard_granularity=4096'/>
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<qemu:arg value='-set'/>
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<qemu:arg value='block.scsi0-0-1.discard_granularity=65536'/>
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</qemu:commandline>
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</domain>
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.. index:: Ceph Block Device; cache options
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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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.. _Snapshots: ../rbd-snapshot/
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.. _Installation: ../../install |