Re: [Qemu-devel] block/vpc: Fix vhd extra sectors issue

[Max Reitz <mreitz@redhat.com>]

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On 16.11.2015 21:58, Max Reitz wrote:
> On 13.11.2015 16:32, Lucian Petrut wrote:
>>
>> At the moment, qemu-img extends new image virtual sizes based
>> on the CHS algorithm provided by the VHD specs in order to
>> ensure that the disk geometry (and payload as seen by some
>> guests which use the CHS value) can fit in the requested disk.
>>
>> This patch drops this behavior, as it breaks compatibility with
>> Azure, which requires the MB alignment to be preserved.
>>
>> Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>
>> ---
>> Proposed fix for https://bugs.launchpad.net/qemu/+bug/1490611
>>
>>  block/vpc.c | 32 ++++++++------------------------
>>  1 file changed, 8 insertions(+), 24 deletions(-)
>>
> 
> CC-ing Peter.
> 
> Looks right, considering page 7 of the spec:
> 
> (http://download.microsoft.com/download/f/f/e/ffef50a5-07dd-4cf8-aaa3-442c0673a029/Virtual%20Hard%20Disk%20Format%20Spec_10_18_06.doc)
> 
> "When the user creates a hard disk of a certain size, the size of the
> hard disk image in the virtual machine is smaller than that created by
> the user. This is because CHS value calculated from the hard disk size
> is rounded down."
> 
> However, this patch looks incomplete, note the comment you are removing:
> "This ensures that qemu-img convert doesn't truncate images". This is
> because vpc_open() prefers the CHS size over the current_size value, and
> as far as I remember there was indeed a good reason for that (some
> application had invalid values for current_size, or something like that).
> 
> So at the very least, we will have to change vpc_open() to always use
> the value from footer->current_size and ignore
> footer->{cyls,heads,secs_per_cyl}, otherwise qemu will always consider
> these images smaller than they actually are (which is bad, at least when
> converting images).
> 
> But I'd like to know from Peter whether he remembers the reason why
> vpc_open() tries to ignore footer->current_size in the first place...

OK, so it was just qemu-devel having hiccups again. Now the mails are
rolling in...

> Max
> 
> 
> PS: If possible, the "From: " header in an emailed patch should match
> the Signed-off-by line. I think. It doesn't state that in
> http://wiki.qemu.org/Contribute/SubmitAPatch, but the commit looks
> strange otherwise. So I guess if it isn't reasonably possible, it's not
> too bad. :-)
> 



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