[Qemu-devel] Re: [PATCH] vdi: Fix image opening and creation for odd disk sizes

[Stefan Weil <weil@mail.berlios.de>]

Am 07.05.2010 13:55, schrieb François Revol:
> Le Fri, 07 May 2010 09:55:23 +0200, Kevin Wolf a écrit :
>> Am 06.05.2010 20:29, schrieb Stefan Weil:
>>> This patch fixes a regression introduced by commit
>>> 95a2f9bc588c3f83375d87b0a9394f89a1bcfada.
>>>
>>> The fix is based on a patch from Kevin Wolf. Here his comment:
>>>
>>> "The number of blocks needs to be rounded up to cover all of the
>>> virtual hard
>>> disk. Without this fix, we can't even open our own images if their
>>> size is not
>>> a multiple of the block size."
>>>
>>> While Kevin's patch addressed vdi_create, my modification also
>>> fixes
>>> vdi_open which now accepts any image which is large enough to hold
>>> the blocks.
>>
>> Shouldn't it be the other way round? That is, an image which has some
>> unused blocks at its end makes sense, whereas an image with a virtual
>> disk size that can't be represented with the number of blocks
>> doesn't?
>
> Exactly, else you don't create what you are asked for.
>
>>> I also decided to keep the original code in vdi_create which rounds
>>> down.
>>> Rounding works in both directions, and there are good arguments for
>>> both,
>>> so I just left the original simple code.
>>>
>>> It is very important to use the rounded value for the new disk
>>> size, too -
>>> otherwise VirtualBox cannot open our disk image.
>>
>> So you're saying that in VDI you can't represent disks with an odd
>> size?
>> The one thing common across image formats seems to be that they are
>> broken...
>
> VB works quite well with my converted laptop image which indeed doesn't
> end on block boundary.
>
> Was it because you were just setting size larger than the covered by
> the blocks ?
>
> François.

Kevin and you are right, and my interpretation of disk_size was wrong.

disk_size is not the size used for the blocks (then it would have to be
large enough to keep all blocks).

disk_size is the number of bytes which are really used for data
(so it is less or equal blocks_in_image * 1 MiB). VBoxManage
allows creation of disk images which use the last block only partially,
something I did not know up to now.

Kevin's patch is correct but still incomplete. VBoxManage can
create images with really odd disk sizes (even sizes which are not
a multiple of the sector size), so the checks in vdi_open
need modifications. The current code also fails for read or write
access beyond the last block.

So I'll send a new patch...

Regards
Stefan