Re: [Qemu-devel] [Qemu-trivial] [PATCH] Teach block/vdi about "discarded" (no longer allocated) blocks

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Oct 26, 2011 at 03:51:18PM -0400, Eric Sunshine wrote:
> An entry in the VDI block map will hold an offset to the actual block if
> the block is allocated, or one of two specially-interpreted values if
> not allocated. Using VirtualBox terminology, value VDI_IMAGE_BLOCK_FREE
> (0xffffffff) represents a never-allocated block (semantically arbitrary
> content).  VDI_IMAGE_BLOCK_ZERO (0xfffffffe) represents a "discarded"
> block (semantically zero-filled).  block/vdi knows only about
> VDI_IMAGE_BLOCK_FREE.  Teach it about VDI_IMAGE_BLOCK_ZERO.
> 
> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
> ---
> 
> Without this patch, "qemu-image check" on a VDI image containing
> discarded blocks reports errors such as:
> 
>   ERROR: block index 3434 too large, is 4294967294
> 
> Decimal 4294967294 is 0xfffffffe. Worse, "qemu-image convert" or direct
> access of the VDI image from qemu involves reads and writes of blocks at
> the bogus block offset 4294967294 within the image file.
> 
> Cc: Stefan Weil <weil@mail.berlios.de>
> Cc: Kevin Wolf <kwolf@redhat.com>
> 
>  block/vdi.c |   23 ++++++++++++++---------
>  1 files changed, 14 insertions(+), 9 deletions(-)

Good to see this improvement.  I recently talked to a CernVM developer
who had issues with vdi images.  This may fix the issue they were
seeing.

I think Kevin should take this through the block tree.  I won't apply it
to trivial-patches.

Stefan