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

From: Stefan Weil
Subject: [Qemu-devel] Re: [PATCH] vdi: Fix image opening and creation for odd disk sizes
Date: Sun, 09 May 2010 12:17:19 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100411 Iceowl/1.0b1 Icedove/3.0.4

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

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
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

Exactly, else you don't create what you are asked for.

I also decided to keep the original code in vdi_create which rounds
Rounding works in both directions, and there are good arguments for
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
The one thing common across image formats seems to be that they are

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 ?


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...


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