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QEMU: Convert non-Sparse Image to qcow/qcow2 Sparse Image

December 15, 2014 Leave a comment

Convert non-Sparse Image to qcow/qcow2 Sparse Image

While this is not always ideal, I’ve found it handy to build qcow/qcow2 images (for use in OpenStack) by converting a vmdk on FreeBSD. Often times, VMWare images are created with thick provisioning meaning that the disk consumed on the hypervisor will equal the disk allocated to the virtual though there may be an overwhelming quantity of contiguous zero bytes. ¬†For example, creating a thick provisioned disk for a VM of, say, 20GB will result in approximately 20GB being consumed on the hypervisor’s disk even though, perhaps, only 50% of that is in use. The remaining 50% will be zero’d bytes and, while technically empty, still consuming space.

VMDK’s can be converted to qcow/qcow2 using qemu-img(1) similarly to:

qemu-img convert -f vmdk -O qcow2 freebsd-10.0.0-amd64.vmdk freebsd-10.0.0-amd64.qcow2

Excellent, we now have qcow2 image suitable for use within OpenStack. However, the size of this file is likely to be the same or very similar to the original vmdk. This can be taken a step further to significantly reduce the amount of space the file consumes using a sparse file.

Using the ‘-S’ command line option enables us to ensure the output file becomes sparse, therefore freeing the space it consumed with a series of zero bytes. This option, while it defaults to 4k, accepts an argument* (size) permitting users to specify the quantity of bytes which must be zeros before it can be that portion is made “sparse”.

The resulting command line is very similar to the above, except that it includes ‘-S’ and the argument, for example:

qemu-img convert -f vmdk -O qcow2 -S 4k freebsd-10.0.0-amd64.vmdk freebsd-10.0.0-amd64.qcow2

* NOTE: if installed from FreeBSD Ports, the argument was required despite having a default defined.

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Data and information described on this blog are for informational purposes only. The author of this blog provides no warranty/guarantee, expressed or implied, that this data and information will function as described here. Readers are expected to exercise due diligence when researching, developing, and deploying techniques and methods for use within their environments.

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Categories: Technical Miscellany