vitalif
/
vitastor
5
4
Fork 9
Code Issues 1 Pull Requests 2 Actions Releases 68 Wiki Activity
vitastor/docs/usage/qemu.en.md

3.2 KiB
Raw Blame History

Documentation → Usage → QEMU and qemu-img

Читать на русском

QEMU and qemu-img

QEMU

You need patched QEMU version to use Vitastor driver. Pre-built packages are available.

To start a VM using plain QEMU command-line with Vitastor disk, use the following commands:

Old syntax (-drive):

qemu-system-x86_64 -enable-kvm -m 1024 \
    -drive 'file=vitastor:etcd_host=192.168.7.2\:2379/v3:image=debian9',
        format=raw,if=none,id=drive-virtio-disk0,cache=none \
    -device 'virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,
        id=virtio-disk0,bootindex=1,write-cache=off' \
    -vnc 0.0.0.0:0

New syntax (-blockdev):

qemu-system-x86_64 -enable-kvm -m 1024 \
    -blockdev '{"node-name":"drive-virtio-disk0","driver":"vitastor","image":"debian9",
        "cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}' \
    -device 'virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,
        id=virtio-disk0,bootindex=1,write-cache=off' \
    -vnc 0.0.0.0:0

qemu-img

For qemu-img, you should use vitastor:etcd_host=<HOST>:image=<IMAGE> as filename.

For example, to upload a VM image into Vitastor, run:

qemu-img convert -f qcow2 debian10.qcow2 -p -O raw 'vitastor:etcd_host=192.168.7.2\:2379/v3:image=debian10'

You can also specify :pool=<POOL>:inode=<INODE>:size=<SIZE> instead of :image=<IMAGE> if you don't want to use inode metadata.

Exporting snapshots

Starting with 0.8.4, you can also export individual layers (snapshot diffs) using qemu-img.

Suppose you have an image testimg and a snapshot testimg@0 created with vitastor-cli snap-create testimg@0.

Then you can export the testimg@0 snapshot and the data written to testimg after creating the snapshot separately using the following commands (key points are using skip-parents=1 and -B backing_file option):

qemu-img convert -f raw 'vitastor:etcd_host=192.168.7.2\:2379/v3:image=testimg@0' \
    -O qcow2 testimg_0.qcow2

qemu-img convert -f raw 'vitastor:etcd_host=192.168.7.2\:2379/v3:image=testimg:skip-parents=1' \
    -O qcow2 -o 'cluster_size=4k' -B testimg_0.qcow2 testimg.qcow2

In fact, with cluster_size=4k any QCOW2 file can be used instead -B testimg_0.qcow2, even an empty one.

QCOW2 cluster_size=4k option is required if you want testimg.qcow2 to contain only the data overwritten exactly in the child layer. With the default 64 KB QCOW2 cluster size you'll get a bit of extra data from parent layers, i.e. a 4 KB overwrite will result in testimg.qcow2 containing 64 KB of data. And this extra data will be taken by qemu-img from the file passed in -B option, so you really need 4 KB cluster if you use an empty image in -B.

After this procedure you'll get two chained QCOW2 images. To detach testimg.qcow2 from its parent, run:

qemu-img rebase -u -b '' testimg.qcow2

This can be used for backups. Just note that exporting an image that is currently being written to is of course unsafe and doesn't produce a consistent result, so only export snapshots if you do this on a live VM.