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KVM convert qcow2 disk images to raw disk images for performance
Published: 16-02-2014 | Author: Remy van Elst | Text only version of this article
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Table of Contents
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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For me it has a performance boost. Using a RAID 1 setup with two 5900 RPM disks
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:
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
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
disk section to point to the new raw image:
Change the lines
<driver name='qemu' type='qcow2'/> and
name='qemu' type='raw'/> and
kvm/tmp20ePgc.raw'/>. 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:
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:
noop [deadline] cfq
deadline scheduler is being used. To change it:
echo noop > /sys/block/sda/queue/scheduler
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
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash elevator=noop" and run an
grub2 to make it permanent