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Re: [Qemu-discuss] QCOW2 real hard drive image

From: Jakob Bohm
Subject: Re: [Qemu-discuss] QCOW2 real hard drive image
Date: Tue, 28 Aug 2012 14:50:25 +0200
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On 8/27/2012 6:52 PM, Mike Lovell wrote:
On 08/18/2012 02:03 AM, Jerzy Grzegorek wrote:

I'd like to create the smallest possible qcow2 (or other format) real hard drive image. It is possible to create it without "dirty" no used sectors in the way like this (with additional options)

qemu-img convert -f raw /dev/sda -O qcow2 disk.qcow2.img

unfortunately, i don't think qemu-img can determine whether or not a sector is 'dirty.' i'm guessing by 'dirty' you mean a sector that has had data written to it but the file system on top no longer needs the data because the file was removed or truncated or similar. determining which sector is 'dirty' in this case would depend on knowing what filesystem is used and what parts of the disk the filesystem currently has allocated and what is free space. i don't think qemu-img can do this.

you could try just adding -c to the command you listed and qemu-img will create a compressed qcow2. that could reduce the size of the resulting image significantly. other than that, compressing it further would take additional steps.


An alternative may be to find a filesystem specific tool which can filter out the unused sectors.

One possible tool (for ntfs) is the ntfsdump utility from ntfsprogs, it may be able to do this with selected options.

Another possible tool is Partimage from partimage.org. It can copy a disk skipping unused sectors for many file systems. To use it you would need to attach an empty qcow2 image of proper size as a virtual disk (e.g. using NMB) and then tell Partimage to do the cloning.

Another alternative is first create an uncompressed disk image, mount it and run a free space wiping tool, unmount it and then tell qemu-img to create an image from it.

All 3 approaches involve using file system specific tools to figure out which disk sectors are unused.


The phrase "dirty sector" normally refers to a sector whose data has not been completely saved to disk yet. Which is kind of the opposite of an unused sector.



Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
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