======================== 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 .. 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