How to: Backup of KVM guests when using qcow2 disk files #dev #it #development

Backup of KVM guests when using qcow2 disk files

I got a Ubuntu (hardy) system used for virtualization using KVM. Each guest got it’s own qcow2 file used as disk. Guests are running Ubuntu (hardy). What would a good backup strategy be for such a setup? I would prefer not to shutdown the guests during backup.

Answer [by Tim Post]: Backup of KVM guests when using qcow2 disk files

You’ll want to…

Resolved: free up not used space on a qcow2-image-file on kvm/qemu #answer #programming #it

free up not used space on a qcow2-image-file on kvm/qemu

we are using kvm/qemu with qcow2-images for our virtual machines.

qcow2 has this nice feature where the image file only allocates the actually needed space by the virtual-machine. but how do i shrink back the image file, if the virtual machine’s allocated space gets smaller?

example:

1.) i create a new image with qcow2 format, size 100GB

2…

Fix: Incredibly slow KVM disk performance (qcow2 disk files + virtio) #it #dev #solution

Incredibly slow KVM disk performance (qcow2 disk files + virtio)

I’m having some serious disk performance problems while setting up a KVM guest. Using a simple dd test, the partition on the host that the qcow2 images reside on (a mirrored RAID array) writes at over 120MB/s, while my guest gets writes ranging from 0.5 to 3MB/s.

The guest is configured with a couple of CPUs and 4G of RAM and isn’t…

How to: Incredibly slow KVM disk performance (qcow2 disk files + virtio) #programming #dev #development

Incredibly slow KVM disk performance (qcow2 disk files + virtio)

I’m having some serious disk performance problems while setting up a KVM guest. Using a simple dd test, the partition on the host that the qcow2 images reside on (a mirrored RAID array) writes at over 120MB/s, while my guest gets writes ranging from 0.5 to 3MB/s.

The guest is configured with a couple of CPUs and 4G of RAM and isn’t…

KVM のディスクイメージファイルは QCOW2 形式で作っておくと qemu-img コマンドでファイルサイズを圧縮できるみたい。 まず、圧縮をかける前にディスクイメージを使っている仮想マシン (samplevm) をあらかじめシャットダウンする。 もし、仮想マシンが ACPI に対応していないときは直接コンソールにログインして落とす。

$ sudo virsh shutdown samplevm

シャットダウンできたら qemu-img convert コマンドに -c オプションを付けてディスクイメージファイルを圧縮する。

$ sudo qemu-img convert -c -f qcow2 -O qcow2 /var/lib/libvirt/images/samplevm{,.compressed}.qcow2

圧縮前と圧縮後のファイルサイズを見ると 1/3 近くまで圧縮できたようだ。

$ ls -alFh /var/lib/libvirt/images/ | grep samplevm -rw-r--r--. 1 root root 472M 1月 25 04:48 2014 samplevm.compressed.qcow2 -rw-r--r--. 1 root root 1.3G 1月 25 04:32 2014 samplevm.qcow2

次に圧縮後のディスクイメージファイルから仮想マシンを起動する。 圧縮後のファイルを圧縮前のファイルにリネームしちゃってもいいんだけど、今回は Libvirt のドメイン定義ファイルを編集する。 ドメイン定義ファイルの編集には virsh edit コマンドを使う。

$ sudo virsh edit samplevm

編集内容は、現在使用している圧縮前のファイルのパスを圧縮後のものに書き換えれば良い。

<source file='/var/lib/libvirt/images/samplevm.qcow2'/> ↓ <source file='/var/lib/libvirt/images/samplevm.compressed.qcow2'/>

編集が終わったら仮想マシンを立ち上げる。 無事起動してコンソールが取れれば成功だ。

$ sudo virsh start samplevm $ sudo virsh console samplevm ...(省略)... Starting sshd: [ OK ] Starting postfix: [ OK ] Starting crond: [ OK ] CentOS release 6.5 (Final) Kernel 2.6.32-431.el6.x86_64 on an x86_64

Resizing a VM partition can be tricky, so if you've ever created a disk image and realised that you needed more space, this post explains how you can resize a QEMU virtual disk, and expand its partitions if you need.

I don't recommend trying to resize the partition in place with truncate, so in this example we will create a new disk image and copy over the contents from our existing VM's disk image, resizing the partitions in the process. Our VM domain name is herman, and it's a QEMU disk image - qcow2. 

First, create a temp directory where you will be working in:

TMPDIR=`mktemp -d` cd $TMPDIR

Then create your new disk here with qemu-img, with the desired size. In my case, I've chosen 32GB:

$ qemu-img create -f qcow2 -o preallocation=metadata $TMPDIR/herman_vm.qcow2 32GB

Formatting '/tmp/tmp.NKT2q5rDVz/herman_vm.qcow2', fmt=qcow2 size=34359738368 encryption=off cluster_size=65536 preallocation='metadata'

Allegedly preallocation=metadata improves I/O performance, and for a little more performance you can try fallocate. Take the size in bytes from the feedback in the previous command use fallocate if you want to preallocate the disk space:

$ fallocate -l 34359738368 $TMPDIR/herman_vm.qcow2

Clone your existing disk into the new one

Now you have your new disk ready, it's time to copy over your existing disk's data.

To find out where your existing disk image is, use virsh dumpxml and query the xml path.

$ sudo virsh dumpxml herman | xpath -e /domain/devices/disk/source > Found 1 nodes in stdin: > -- NODE -- > <source file="/vms/herman/tmpkdYhWL.qcow2" /> $ OLD_DISK=/vms/herman/tmpkdYhWL.qcow2

It is usefull to know how your existing partitions are layed out. You can find that out with virt-filesystems:

$ sudo virt-filesystems --long --parts --blkdevs -h -a $OLD_DISK

Note down the partitions you want to expand, or resize: expand means take up the remaining free space (only one partition can be expanded), while resize means resize to a specific size. If you resize all your partitions, the remaining free space will be unallocated.

In our case, I've chosen to resize the swap /dev/sda2 partition to 2GB and expand the / partition (which will be aproximately 30GB).

$ sudo virt-resize --resize /dev/sda2=+1G --expand /dev/sda1 $OLD_DISK $TMPDIR/herman_vd.qcow2

After a long output copying over the data in your existing virtual disk, the partition will be resize. Move the file to the vm's folder, and remember to update permissions (yours may differ from mine):

$ sudo mv $TMPDIR/herman_vd.qcow2 /vms/herman/ $ sudo chown libvirt-qemu.kvm /vms/herman/herman_vd.qcow2

You're almost done now. You need to update the xml config for your VM:

$ sudo virsh edit herman

and change <source file="/vms/herman/tmpkdYhWL.qcow2" /> under devices/disk to <source file="/vms/herman/herman_vd.qcow2" />

Restart the VM

After you're done, you can delete the TMPDIR and restart the VM and test if it's working

$ rmdir $TMPDIR $ sudo virsh destroy herman $ sudo virsh start herman