Re: [Qemu-devel] [RFC PATCH] vpc: Ignore geometry for large images

[Peter Lieven <pl@kamp.de>]

Am 09.02.2015 um 17:09 schrieb Kevin Wolf:
> The CHS calculation as done per the VHD spec imposes a maximum image
> size of ~127 GB. Real VHD images exist that are larger than that.
>
> Apparently there are two separate non-standard ways to achieve this:
> You could use more heads than the spec does - this is the option that
> qemu-img create chooses.
>
> However, other images exist where the geometry is set to the maximum
> (65536/16/255), but the actual image size is larger. Until now, such
> images are truncated at 127 GB when opening them with qemu.
>
> This patch changes the vpc driver to ignore geometry in this case and
> only trust the size field in the header.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>
> Peter, I'm replacing some of your code in the hope that the new approach
> is more generally valid. Of course, I haven't tested if your case with
> disk2vhd is still covered. Could you check this, please?

I checked this and found that disk2vhd always sets CHS to 65535ULL * 16 * 255
independed of the real size.

But, as the conversion to CHS may have an error its maybe the best solution
to ignore CHS completely and always derive total_sectors from footer->size unconditionally.

I had a look at what virtualbox does and they only rely on footer->size. If they
alter the size or create an image the write the new size into the footer and recalculate
CHS by the formula found in the appendix of the original spec.

Check vhdCreateImage, vhdOpen in http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Storage/VHD.cpp

The original spec also says that CHS values purpose is the use in an ATA controller only.

Peter


>
>   block/vpc.c | 10 ++++------
>   1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/block/vpc.c b/block/vpc.c
> index 46803b1..00409a6 100644
> --- a/block/vpc.c
> +++ b/block/vpc.c
> @@ -215,12 +215,10 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>       bs->total_sectors = (int64_t)
>           be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
>   
> -    /* images created with disk2vhd report a far higher virtual size
> -     * than expected with the cyls * heads * sectors_per_cyl formula.
> -     * use the footer->size instead if the image was created with
> -     * disk2vhd.
> -     */
> -    if (!strncmp(footer->creator_app, "d2v", 4)) {
> +    /* Images that have exactly the maximum geometry are probably bigger and
> +     * would be truncated if we adhered to the geometry for them. Rely on
> +     * footer->size for them. */
> +    if (bs->total_sectors == 65535ULL * 16 * 255) {
>           bs->total_sectors = be64_to_cpu(footer->size) / BDRV_SECTOR_SIZE;
>       }
>   


-- 

Mit freundlichen Grüßen

Peter Lieven

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