Red Hat Bugzilla – Bug 528678
qemu-img does not correctly roundtrip data for VPC image format
Last modified: 2009-10-13 15:40:40 EDT
Description of problem:
If you create a raw file, convert it to VPC format with qemu-img, and then convert it back to raw format, the resulting raw file is not the same as the original. All the other disk formats roundtrip reliably, but not VPC.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. dd if=/dev/zero of=input.img bs=1M count=50
2. qemu-img convert -f raw -O vpc input.img output.img
3. qemu-img convert -f vpc -O raw output.img input2.img
4. md5sum input.img
5. md5sum input2.img
6. ls -l input.img
7. ls -l input2.img
-rw-rw-r--. 1 berrange berrange 52428800 2009-10-13 11:44 input.img
-rw-r--r--. 1 berrange berrange 52432896 2009-10-13 11:45 input2.img
input.img and input2.img should be identical in everyway.
Created attachment 364572 [details]
Example failure when testing libvirt cloning API
FYI, the original way we discovered this bug was via the libvirt TCK's storage cloning tests. That tests creates a raw volume with a special data pattern, and then tests conversion to & from every QEMU writable disk format. We currently have to mark the VPC tests as TODO, since we get md5sum mis-matches after VPC -> raw conversion with unexpected extra NULL bytes appended on the final raw file.
kevin: sounds like something you could sort out quickly if you had some time to spare :-)
This is known problem, but I think this is not a qemu bug but a shortcoming of the VHD format (or VirtualPC's handling of that format) where we are doomed to do it wrong at some place. Adding zeros seemed to be the least wrong thing when I implemented it.
For reference, I explained the behaviour in http://lists.gnu.org/archive/html/qemu-devel/2009-08/msg00570.html:
> I think this depends on the definition of "correct". VPC unfortunately
> seems to like contradicting values in its image - CHS geometry and image
> size rarely match. The guest seemed to see the CHS geometry, so I
> changed the code to take the geometry instead of the size header field
> to calculate the image size.
If we didn't round up to the next matching CHS values and used the exact value, our images would seem to be truncated in VirtualPC. You shouldn't observe the problem when you choose the image size so that its size can be represented exactly in a CHS geometry.
Worked around the patch in this commit, by adding 4k to the disk size to make it a nice size for CHS geom