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126 lines
5.0 KiB
Markdown
126 lines
5.0 KiB
Markdown
[Documentation](../../README.md#documentation) → Usage → QEMU and qemu-img
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-----
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[Читать на русском](qemu.ru.md)
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# QEMU and qemu-img
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## QEMU
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You need patched QEMU version to use Vitastor driver. Pre-built [packages](../installation/packages.en.md) are available.
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To start a VM using plain QEMU command-line with Vitastor disk, use the following commands:
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Old syntax (-drive):
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```
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qemu-system-x86_64 -enable-kvm -m 1024 \
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-drive 'file=vitastor:etcd_host=192.168.7.2\:2379/v3:image=debian9',
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format=raw,if=none,id=drive-virtio-disk0,cache=none \
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-device 'virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,
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id=virtio-disk0,bootindex=1,write-cache=off' \
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-vnc 0.0.0.0:0
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```
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New syntax (-blockdev):
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```
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qemu-system-x86_64 -enable-kvm -m 1024 \
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-blockdev '{"node-name":"drive-virtio-disk0","driver":"vitastor","image":"debian9",
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"cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}' \
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-device 'virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,
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id=virtio-disk0,bootindex=1,write-cache=off' \
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-vnc 0.0.0.0:0
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```
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## qemu-img
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For qemu-img, you should use `vitastor:etcd_host=<HOST>:image=<IMAGE>` as filename.
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For example, to upload a VM image into Vitastor, run:
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```
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qemu-img convert -f qcow2 debian10.qcow2 -p -O raw 'vitastor:etcd_host=192.168.7.2\:2379/v3:image=debian10'
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```
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You can also specify `:pool=<POOL>:inode=<INODE>:size=<SIZE>` instead of `:image=<IMAGE>`
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if you don't want to use inode metadata.
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### Exporting snapshots
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Starting with 0.8.4, you can also export individual layers (snapshot diffs) using `qemu-img`.
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Suppose you have an image `testimg` and a snapshot `testimg@0` created with `vitastor-cli snap-create testimg@0`.
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Then you can export the `testimg@0` snapshot and the data written to `testimg` after creating
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the snapshot separately using the following commands (key points are using `skip-parents=1` and
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`-B backing_file` option):
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```
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qemu-img convert -f raw 'vitastor:etcd_host=192.168.7.2\:2379/v3:image=testimg@0' \
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-O qcow2 testimg_0.qcow2
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qemu-img convert -f raw 'vitastor:etcd_host=192.168.7.2\:2379/v3:image=testimg:skip-parents=1' \
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-O qcow2 -o 'cluster_size=4k' -B testimg_0.qcow2 testimg.qcow2
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```
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In fact, with `cluster_size=4k` any QCOW2 file can be used instead `-B testimg_0.qcow2`, even an empty one.
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QCOW2 `cluster_size=4k` option is required if you want `testimg.qcow2` to contain only the data
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overwritten **exactly** in the child layer. With the default 64 KB QCOW2 cluster size you'll
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get a bit of extra data from parent layers, i.e. a 4 KB overwrite will result in `testimg.qcow2`
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containing 64 KB of data. And this extra data will be taken by `qemu-img` from the file passed
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in `-B` option, so you really need 4 KB cluster if you use an empty image in `-B`.
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After this procedure you'll get two chained QCOW2 images. To detach `testimg.qcow2` from
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its parent, run:
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```
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qemu-img rebase -u -b '' testimg.qcow2
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```
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This can be used for backups. Just note that exporting an image that is currently being written to
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is of course unsafe and doesn't produce a consistent result, so only export snapshots if you do this
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on a live VM.
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## VDUSE
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Linux kernel, starting with version 5.15, supports a new interface for attaching virtual disks
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to the host - VDUSE (vDPA Device in Userspace). QEMU, starting with 7.2, has support for
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exporting QEMU block devices over this protocol using qemu-storage-daemon.
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VDUSE has the same problem as other FUSE-like interfaces in Linux: if a userspace process hangs,
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for example, if it loses connectivity with Vitastor cluster - active processes doing I/O may
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hang in the D state (uninterruptible sleep) and you won't be able to kill them even with kill -9.
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In this case reboot will be the only way to remove VDUSE devices from system.
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On the other hand, VDUSE is faster than [NBD](nbd.en.md), so you may prefer to use it if
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performance is important for you. Approximate performance numbers:
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direct fio benchmark - 115000 iops, NBD - 60000 iops, VDUSE - 90000 iops.
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To try VDUSE you need at least Linux 5.15, built with VDUSE support
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(CONFIG_VIRTIO_VDPA=m and CONFIG_VDPA_USER=m). Debian Linux kernels have these options
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disabled by now, so if you want to try it on Debian, use a kernel from Ubuntu
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[kernel-ppa/mainline](https://kernel.ubuntu.com/~kernel-ppa/mainline/) or Proxmox.
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Commands to attach Vitastor image as a VDUSE device:
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```
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modprobe vduse virtio-vdpa
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qemu-storage-daemon --daemonize --blockdev '{"node-name":"test1","driver":"vitastor",\
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"etcd-host":"192.168.7.2:2379/v3","image":"testosd1","cache":{"direct":true,"no-flush":false},"discard":"unmap"}' \
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--export vduse-blk,id=test1,node-name=test1,name=test1,num-queues=16,queue-size=128,writable=true
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vdpa dev add name test1 mgmtdev vduse
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```
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After running these commands /dev/vda device will appear in the system and you'll be able to
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use it as a normal disk.
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To remove the device:
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```
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vdpa dev del test1
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kill <qemu-storage-daemon_process_PID>
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```
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