I have recently begun experimenting with the KVM virtualization infrastructure. After deployment, I used virt-manager running on a management computer to build my first guest. I notice that, by default, the resulting virtual machine is in the RAW format.

From what I understand, it is possible to create virtual disks of various formats. (http://www.linux-kvm.org/page/FAQ#What_virtual_disk_formats_can_KVM_use.3F).

My questions are: 1) What format is generally considered most portable? (i.e. moving to a new virtualization platform/hypervisor)

2) Is is possibly to specify format during virtual machine creation in virt-manager, if so, how?

3) Can anyone suggest a more feature rich management tool besides virt-manager?


virt-manager creates raw disk images by default. These are the most portable, since any hypervisor known to man can deal with them. If you create one as a file on disk, it will have the .img extension.

If you want to choose a different format, then when adding storage, choose "Select managed or other existing storage", then click Browse. On the next screen select a storage pool, click New Volume, and you can create a volume in any of a variety of formats.

Storage configuration

As for other management tools, you have a huge variety, everything from OpenStack to the virsh command line tool. You'll have to do your own shopping here....


.img is just an extension, you can call the file disk.disk or whatever else. The default format is what matters.

The most common format is RAW - it has the best access speeds because it is what it's called - just direct mapping of blocks, nothing else. This also means no extra features.

The arguably second most common format is QCOW2, this is a bit slower than RAW, but it can provide encryption, zero deduplication, snapshots, thin provisioning and additional goodies.

In addition, you can usually combine the two, the most common use case is to have a RAW image at he base of the snapshot chain, and multiple QCOW2 formatted snapshots daisychained off of it.

  • Your final point is a point of interest for me. Can you elaborate (or point me to some resources) for how to couple RAW with QCOW in the manner that you suggest? This methodology seems to offer the best features of both formats. – sardean Feb 1 '14 at 0:06
  • It's as simple as creating a raw formatted disk, and then taking a snapshot of it. The snapshot will automatically be qcow2 based – dyasny Feb 1 '14 at 0:15

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