Re: [Qemu-devel] [PATCH 5/7] block/vpc: Fix size calculation

[Stefan Weil]

Am 05.02.2013 11:54, schrieb Kevin Wolf:
> Am 01.02.2013 22:51, schrieb Stefan Weil:
>> The size calculated from the CHS values is not the real image (disk) size,
>> but usually a smaller value (this is caused by rounding effects).
>>
>> Only older operating systems use CHS. Such guests won't be able to use
>> the whole disk.
>>
>> This patch fixes https://bugs.launchpad.net/qemu/+bug/1105670/.
>>
>> Signed-off-by: Stefan Weil <address@hidden>
>> ---
>>  block/vpc.c |    9 ++++-----
>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/vpc.c b/block/vpc.c
>> index fff103b..799b1c9 100644
>> --- a/block/vpc.c
>> +++ b/block/vpc.c
>> @@ -198,11 +198,10 @@ static int vpc_open(BlockDriverState *bs, int flags)
>>      /* Write 'checksum' back to footer, or else will leave it with zero. */
>>      footer->checksum = be32_to_cpu(checksum);
>>  
>> -    // The visible size of a image in Virtual PC depends on the geometry
>> -    // rather than on the size stored in the footer (the size in the footer
>> -    // is too large usually)
>> -    bs->total_sectors = (int64_t)
>> -        be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
>> +    /* The visible size of an image in Virtual PC may depend on the geometry
>> +     * rather than on the size stored in the footer (the size in the footer
>> +     * is usually larger). Nevertheless we must use the real size here. */
>> +    bs->total_sectors = be64_to_cpu(footer->size) / 512;
>>  
>>      /* Allow a maximum disk size of approximately 2 TB */
>>      if (bs->total_sectors >= 65535LL * 255 * 255) {
> Now VMs created with qemu can't be run in Virtual PC any more because
> they use more space than is really visible on VPC.

Now VMs created with QEMU show the size which is also
reported by MS tools. I don't think this is a problem.


> The bug report doesn't say anything about practical consequences of the
> unexpected behaviour, so I'd tend to claim that just the expectation was
> wrong. Having to deal with two differing sizes is ugly, but we had the
> discussion before and using the geometry was considered the least bad
> way, because it gives you the same disk size within the VM as on Virtual PC.
>
> If you want to change it, you need to provide a real use case that breaks.
>
> Kevin

Test case:

* Create a VHD disk image (either with QEMU or MS tools).
* Run a modern guest OS (which does not use CHS) on this image
  using Virtual PC or VirtualBox and write into the last disk blocks.
* Copy the image to another format using qemu-img.

=> The last blocks are missing now.

Stefan