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[Qemu-devel] [Bug 660366] Re: "qemu-img convert -O qcow2 -o backing_file

From: Stefan Hajnoczi
Subject: [Qemu-devel] [Bug 660366] Re: "qemu-img convert -O qcow2 -o backing_file" makes huge images
Date: Thu, 14 Oct 2010 09:25:46 -0000

It is possible to chain backing files.  As a workaround you could do the

$ qemu-img create -f qcow2 -b 2.img 4.img # from now on don't modify 2.img, 
instead use 4.img
$ qemu-img create -f qcow2 -b 2.img 3.img # here is the 3.img you tried to 
create with qemu-convert

Images 1.img and 2.img should never be modified, they are immutable

Images 3.img and 4.img can be modified and will contain only changes
against 2.img.

Perhaps qemu-img needs a command to drop data that is duplicated in the
base image.  This could be a new flag to commit: qemu-img commit --dedup

"qemu-img convert -O qcow2 -o backing_file" makes huge images
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Status in QEMU: New

Bug description:
$ dd if=/dev/urandom bs=1M of=1.img count=4
4+0 records in
4+0 records out
4194304 bytes (4,2 MB) copied, 1,0413 s, 4,0 MB/s
$ qemu-img create -f qcow2 -b 1.img 2.img
Formatting '2.img', fmt=qcow2 size=4194304 backing_file='1.img' encryption=off 
$ qemu-img convert -O qcow2 -o backing_file=1.img 2.img 3.img
$ du -h ?.img
4,1M    1.img
144K    2.img
4,3M    3.img

The conversion result is bigger then the source!

It appears that "-o backing_file" is not applied to data (as expected). I.e. 
all data is put into the resulting image: both from source image and "backing" 

Expected behavior is to put only data that is not present in backing_file.

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