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[Qemu-devel] Growing qcow images

From: Ben Taylor
Subject: [Qemu-devel] Growing qcow images
Date: Thu, 23 Jun 2005 10:46:53 -0400

I was looking at the differences between a 1G and 2G
initial qcow formatted image, and found that there 
wasn't much difference. Just a a size byte for the
image, along with a difference in 4K of zeros at the
end of the file. (This seems to be consistent for
every increment of 1G at initial creation time)

Test #1) copy the 1G image and modify what I thought was
the size byte.  Running qemu-img info against that image
produced an error.  Seems qemu-img wants to read the initial
bytes at the end of the file, even if there's nothing there.

Test #2) copy the 1G image and modify the same byte. DD the
last 4096 bytes from the 2G image and append it to the new
1G image.  qemu-img info now recognizes it as a 2G image.

Test #3) copy an existing 2G win98se install at 227MB and
edit the size byte, modifying from 8 to c.  qemu-img info
now recognizes it as a 3G image.  Qemu recognizes it now
as a 3G disk with a 2G image.  running qtparted found 1g
of free space, and I expanded the win98 (fat32) partition
to the maximum size.

Then booted the win98 image and it now sees 3GB of space.

This was just a hack, but it seems to indicate that 
qcow images could be grown on the fly, with the obvious
implications that you also have to grow the file system
that is in the image, or add another partition.


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