[vbox-dev] vhd resize corruption
Alexander.Eichner at oracle.com
Tue Sep 9 08:35:15 UTC 2014
(please keep the topic on the list)
I tested the code by writing random data to it, resizing the image and verifying that the disk content didn’t changed by reading and comparing the data.
Resizing is a quick operation because VBox just relocates the data blocks which would be overwritten by the extended BAT. The data blocks are appended to the image file and
in most cases only one block needs to be relocated. If you still ave the original and the broken VHD image file around can you please mail me the first 5MB of each image so
I can have a look at them?
On 08.09.2014 22:47, Aaron Brice <aaron.brice at datasoft.com> wrote:
> VirtualBox 4.2 was used to create the .vhd, according to the vhd header.
> What do you mean by not able to reproduce, just that it still boots or did you verify the BAT table doesn't overwrite a data block and that the offsets were recalculated correctly?
> When I ran the resize, it completed very quickly (a few seconds) to resize a 40GB file to 60GB capacity. The BAT table is towards the beginning of the file, I'm not sure how you can increase the size of the BAT table so quickly without overwriting data, I would think you'd have to create a new file? It takes several minutes to copy the file..
> On 09/08/2014 04:24 AM, Alexander Eichner wrote:
>> Hi Aaron,
>> sorry for the late answer (was on vacation last week). I’m not able to reproduce the
>> resize corruption you report (I was never able to looking at the mentioned ticket).
>> Which tool did you use to create the original VHD image?
>> Alexander Eichner
>> On 29.08.2014 21:14, Aaron Brice <aaron.brice at datasoft.com> wrote:
>>> So the 40GB .vhd file that contained a Windows 7 VM was running out of
>>> room, and I wanted to resize it.
>>> With VBoxManage 4.3.10 on Ubuntu I ran:
>>> VBoxManage modifyhd --resize 61440
>>> Anyway, long story short it won't boot, gparted live ISO doesn't find a
>>> partition table..
>>> So I started investigating the .vhd file format. There is 512 bytes in
>>> the beginning consisting of a copy of the footer, then 1024 bytes of the
>>> header, then a variably sized BAT (Block Allocation Table), then the
>>> blocks of data. When the max size of the disk changes the BAT has to be
>>> resized to accommodate potential new blocks. It looks like modifyhd
>>> resizes the BAT, but did not shift the BAT pointers or move the data, it
>>> just expanded the BAT into the first data block, wiping out the MBR. I
>>> don't have the original for comparison, but now the first entry in the
>>> BAT is 0xa3 (163). This used to point to the sector just after the BAT,
>>> but now the BAT has expanded (from 20480 4-byte entries to 30720) and
>>> sector 163 is entry 20481 in the table, and the MBR table is all 0xff's..
>>> Relevant bug: https://www.virtualbox.org/ticket/11960
>>> vbox-dev mailing list
>>> vbox-dev at virtualbox.org
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