--- /dev/null
+========================
+ 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.
+
+
+Usage
+=====
+
+The QEMU command line expects you to specify the pool name and image name. You
+may also specify a snapshot name.
+
+QEMU will assume that the Ceph configuration file resides in the default
+location (e.g., ``/etc/ceph/$cluster.conf``) and that you are executing
+commands as the default ``client.admin`` user unless you expressly specify
+another Ceph configuration file path or another user. When specifying a user,
+QEMU uses the ``ID`` rather than the full ``TYPE:ID``. See `User Management -
+User`_ for details. Do not prepend the client type (i.e., ``client.``) to the
+beginning of the user ``ID``, or you will receive an authentication error. You
+should have the key for the ``admin`` user or the key of another user you
+specify with the ``:id={user}`` option in a keyring file stored in default path
+(i.e., ``/etc/ceph`` or the local directory with appropriate file ownership and
+permissions. Usage takes the following form::
+
+ qemu-img {command} [options] rbd:{pool-name}/{image-name}[@snapshot-name][:option1=value1][:option2=value2...]
+
+For example, specifying the ``id`` and ``conf`` options might look like the following::
+
+ qemu-img {command} [options] rbd:glance-pool/maipo:id=glance:conf=/etc/ceph/ceph.conf
+
+.. tip:: Configuration values containing ``:``, ``@``, or ``=`` can be escaped with a
+ leading ``\`` character.
+
+
+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
+
+ <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>
+
+
+.. 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 cache settings (including settings that
+are explicitly set in the Ceph configuration file).
+
+.. note:: Prior to QEMU v2.4.0, if you explicitly set `RBD Cache`_ settings
+ in the Ceph configuration file, your Ceph settings override the QEMU cache
+ 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
+.. _User Management - User: ../../rados/operations/user-management#user
Code Review / stor4nfv.git / blobdiff