Usage

The QEMU command line expects you to specify the Ceph pool and image name. Youmay also specify a snapshot.

QEMU will assume that Ceph configuration resides in the defaultlocation (e.g.,/etc/ceph/$cluster.conf) and that you are executingcommands as the defaultclient.admin user unless you expressly specifyanother Ceph configuration file path or another user. When specifying a user,QEMU uses theID rather than the fullTYPE:ID. SeeUser Management -User for details. Do not prepend the client type (i.e.,client.) to thebeginning of the userID, or you will receive an authentication error. Youshould have the key for theadmin user or the key of another user youspecify 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 andpermissions. Usage takes the following form:

qemu-img{command}[options]rbd:{pool-name}/{image-name}[@snapshot-name][:option1=value1][:option2=value2...]

For example, specifying theid andconf options might look like the following:

qemu-img{command}[options]rbd:glance-pool/maipo:id=glance:conf=/etc/ceph/ceph.conf

Creating Images with QEMU

You can create a block device image from QEMU. You must specifyrbd, thepool name, and the name of the image you wish to create. You must also specifythe size of the image.

qemu-imgcreate-frawrbd:{pool-name}/{image-name}{size}

For example:

qemu-imgcreate-frawrbd:data/foo10G

Resizing Images with QEMU

You can resize a block device image from QEMU. You must specifyrbd,the pool name, and the name of the image you wish to resize. You must alsospecify the size of the image.

qemu-imgresizerbd:{pool-name}/{image-name}{size}

For example:

qemu-imgresizerbd:data/foo10G

Retrieving Image Info with QEMU

You can retrieve block device image information from QEMU. You mustspecifyrbd, the pool name, and the name of the image.

qemu-imginforbd:{pool-name}/{image-name}

For example:

qemu-imginforbd:data/foo

Running QEMU with RBD

QEMU can pass a block device from the host on to a guest, but sinceQEMU 0.15, there’s no need to map an image as a block device onthe host. Instead, QEMU attaches an image as a virtual blockdevice directly vialibrbd. This strategy increases performanceby avoiding context switches and taking advantage ofRBD caching.

You can useqemu-img to convert existing virtual machine images to Cephblock device images. For example, if you have a qcow2 image, you could run:

qemu-imgconvert-fqcow2-Orawdebian_squeeze.qcow2rbd:data/squeeze

To run a virtual machine booting from that image, you could run:

qemu-m1024-driveformat=raw,file=rbd:data/squeeze

RBD caching can significantly improve performance.Since QEMU 1.2, QEMU’s cache options controllibrbd caching:

qemu-m1024-driveformat=rbd,file=rbd:data/squeeze,cache=writeback

If you have an older version of QEMU, you can set thelibrbd cacheconfiguration (like any Ceph configuration option) as part of the‘file’ parameter:

qemu-m1024-driveformat=raw,file=rbd:data/squeeze:rbd_cache=true,cache=writeback

Enabling Discard/TRIM

Since Ceph version 0.46 and QEMU version 1.1, Ceph Block Devices support thediscard operation. This means that a guest can send TRIM requests to let a Cephblock device reclaim unused space. This can be enabled in the guest by mountingext4 orXFS with thediscard option.

For this to be available to the guest, it must be explicitly enabledfor the block device. To do this, you must specify adiscard_granularity associated with the drive:

qemu-m1024-driveformat=raw,file=rbd:data/squeeze,id=drive1,if=none \-devicedriver=ide-hd,drive=drive1,discard_granularity=512

Note that this uses the IDE driver. The virtio driver supports discard since Linux kernel version 5.0.

If using libvirt, edit your libvirt domain’s configuration file usingvirshedit to include thexmlns:qemu value. Then, add aqemu:commandlineblock as a child of that domain. The following example shows how to set twodevices withqemuid= to differentdiscard_granularity values.

<domaintype='kvm'xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'><qemu:commandline><qemu:argvalue='-set'/><qemu:argvalue='block.scsi0-0-0.discard_granularity=4096'/><qemu:argvalue='-set'/><qemu:argvalue='block.scsi0-0-1.discard_granularity=65536'/></qemu:commandline></domain>

QEMU Cache Options

QEMU’s cache options correspond to the following CephRBD Cache settings.

Writeback:

rbd_cache=true

Writethrough:

rbd_cache=truerbd_cache_max_dirty=0

None:

rbd_cache=false

QEMU’s cache settings override Ceph’s cache settings (including settings thatare explicitly set in the Ceph configuration file).