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
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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 `<driver name='qemu' type='qcow2'/>` and `<source
file='/var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.qcow2'/>` to `<driver
name='qemu' type='raw'/>` and `<source file='/var/lib/libvirt/images/vm1/ubuntu-
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:
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
---
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