This is a text-only version of the following page on https://raymii.org: --- Title : KVM convert qcow2 disk images to raw disk images for performance Author : Remy van Elst Date : 16-02-2014 URL : https://raymii.org/s/tutorials/KVM_convert_qcow2_disk_images_to_raw_disk_images_for_performance.html Format : Markdown/HTML --- This tutorial shows you how to convert KVM qcow2 disk images to raw disk images. The qcow2 disk format has some decent features like encryption, compression and copy to write support. However, the compression and the copy processes make it quite a bit slower than raw disk images. Sometimes you want to convert the disk images so that the VM will perform better.

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### Benchmark For me it has a performance boost. Using a RAID 1 setup with two 5900 RPM disks and the `deadline` on the host and the `noop` scheduler without caching on a raw image in the guest resulted in a boost over the default `deadline` scheduler on a qcow2 image on the guest: Test command: dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync Default Ubuntu 12.04 vmbuilder created vm on a qcow2 image without caching and with the deadline scheduler in the VM: 1073741824 bytes (1.1 GB) copied, 658.396 s, 1.6 MB/s The same VM, disk image converted to raw image without caching and using the noop scheduler in the VM: 1073741824 bytes (1.1 GB) copied, 13.646 s, 78.7 MB/s That's quite a performance boost. The KVM host has the following result with the `dd` command: 1073741824 bytes (1.1 GB) copied, 10.4034 s, 103 MB/s ### Converting the image I'll convert the disk image for the example vm `vm1`. Change the name and disk paths for your setup. First shut down the VM: virsh shutdown vm1 Then convert all the disk images using this command for each disk image: qemu-img convert /var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.qcow2 /var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.raw Edit the VM config: virsh edit vm1 Change the `disk` section to point to the new raw image: Change the lines `` and `` to `` and ``. Like so: <disk type='file' device='disk'> <driver name='qemu' type='raw' cache='none'/> <source file='/var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.raw'/> <target dev='hda' bus='ide'/> <address type='drive' controller='0' bus='0' unit='0'/> </disk> That's it. Start the VM: virsh start vm1 If it all works, remove the qcow2 image: rm /var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.qcow2 ### Disk scheduler To change the default disk scheduler in your VM, you can use the following command: echo $scheduler > /sys/block/$device/queue/scheduler To check the current scheduler: cat /sys/block/sda/queue/scheduler Output: noop [deadline] cfq Here the `deadline` scheduler is being used. To change it: echo noop > /sys/block/sda/queue/scheduler Now the `noop` scheduler is being used: cat /sys/block/sda/queue/scheduler [noop] deadline cfq Remember to add this command to `/etc/rc.local` to make it survive a reboot. On ubuntu you can also define it in `/etc/default/grub`. Change `GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"` to `GRUB_CMDLINE_LINUX_DEFAULT="quiet splash elevator=noop"` and run an `update- grub2` to make it permanent [1]: https://www.digitalocean.com/?refcode=7435ae6b8212 --- License: All the text on this website is free as in freedom unless stated otherwise. This means you can use it in any way you want, you can copy it, change it the way you like and republish it, as long as you release the (modified) content under the same license to give others the same freedoms you've got and place my name and a link to this site with the article as source. This site uses Google Analytics for statistics and Google Adwords for advertisements. You are tracked and Google knows everything about you. Use an adblocker like ublock-origin if you don't want it. All the code on this website is licensed under the GNU GPL v3 license unless already licensed under a license which does not allows this form of licensing or if another license is stated on that page / in that software: This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see . Just to be clear, the information on this website is for meant for educational purposes and you use it at your own risk. I do not take responsibility if you screw something up. Use common sense, do not 'rm -rf /' as root for example. If you have any questions then do not hesitate to contact me. See https://raymii.org/s/static/About.html for details.