Are there any benefits of using .qcow2 over .img files when trying to create a VM .

I have a smallish debian .iso and have seen at least with .qcow2 that it is slow.

I also saw http://wiki.qemu.org/Features/Qcow3 and these two lines show up there :-

near-raw performance, competitive with QED

Performance analysis has shown that QCOW2 performance is significantly worse than using raw files. Different approaches, including QED's simplified metadata and fully asynchronous implementation, have proven that a modern image format can achieve near-raw performance. Metadata caching and batched updates can also improve performance but require image format changes to be effective in all cases.

Improving performance is the key motivation for a QCOW3 image format.

Can anybody comment. I am guessing that qcow3 is stil up in the air.

  • Perhaps, a better subject would be: "What are tha main differences between an ISO and a QCO2 image?"
    – sebelk
    Jan 3, 2019 at 14:51

2 Answers 2


A qcow2 file will generally be more space efficient than an equivalent image file. While it is possible to create "sparse" image files, these are difficult to transfer in an efficient manner. A qcow2 relies on mechanisms other than sparse allocation to achieve "thin provisioning" and is thus easier to move around.

A qcow2 file can be defined as a copy-on-write (COW) layer on top of a backing image (which can be either a raw file or another qcow2 image). This can be incredibly useful when you want to boot several virtual machines from the same underlying image, but want each machine to be able to store it's own modifications (a feature called "snapshots"; you can read more details here).

On the other hand, as you have seen, a qcow2 file may not be as performant as a raw image file.

  • hi larsks, img is suppported only by virtual box and qcow2 is used only with virtual manager right... none of these hypervisors support both the formats right?
    – user93868
    Sep 6, 2015 at 3:14
  • 1
    The qcow2 format works primarily with QEMU/KVM, but raw images will work with just about everything (including QEMU/KVM, VMware, VirtualBox, etc).
    – larsks
    Sep 6, 2015 at 3:15

If you don't need specific features of qcow2 and want to use the VM for years, consider not to use qcow2. If you update the operating system frequently, or you have used many temporary files, the deleted files are likely not deallocated on your real hard drive. And you may end up with an image file bigger than raw files.

You can convert between image formats when required, if you have enough free space on your hard drive.

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