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

[Peter Lieven <pl@kamp.de>]

Am 12.02.2015 um 11:06 schrieb Kevin Wolf:
> Am 12.02.2015 um 11:02 hat Peter Lieven geschrieben:
>> Am 12.02.2015 um 10:58 schrieb Kevin Wolf:
>>> Am 12.02.2015 um 10:23 hat Peter Lieven geschrieben:
>>>> Am 10.02.2015 um 15:53 schrieb Kevin Wolf:
>>>>> Am 10.02.2015 um 15:00 hat Peter Lieven geschrieben:
>>>>>> Am 10.02.2015 um 14:54 schrieb Kevin Wolf:
>>>>>>> Am 10.02.2015 um 14:42 hat Jeff Cody geschrieben:
>>>>>>>> On Tue, Feb 10, 2015 at 02:34:14PM +0100, Kevin Wolf wrote:
>>>>>>>>> Am 10.02.2015 um 12:41 hat Peter Lieven geschrieben:
>>>>>>>>>> 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.
>>>>>>>>> The problem with just using footer->size back then when I
>>>>>>>>> implemented this was that from the perspective of a VirtualPC guest
>>>>>>>>> run in qemu, the size of its hard disk would change, which you don't
>>>>>>>>> want either. Going from VPC to qemu would be ugly, but mostly
>>>>>>>>> harmless as the disk only grows. But if you use an image in qemu
>>>>>>>>> where the disk looks larger and then go back to VPC which respects
>>>>>>>>> geometry, your data may be truncated.
>>>>>>>> I believe the vpc "creator" field is different if the image was
>>>>>>>> created by Virtual PC, versus created by Hyper-V ("vpc" and "win",
>>>>>>>> respectively, I think).  Perhaps we could use that to infer a guest
>>>>>>>> image came from VirtualPC, and thus not use footer->size in that
>>>>>>>> scenario?
>>>>>>> Right, I think we discussed that before. Do you remember the outcome of
>>>>>>> that discussion? I seem to remember that we had a conclusion, but
>>>>>>> apparently it was never actually implemented.
>>>>>>>
>>>>>>> Would your proposal be to special-case "vpc" to apply the geometry, and
>>>>>>> everything else (including "win", "d2v" and "qemu") would use the footer
>>>>>>> field?
>>>>>> That sounds reasonable. In any case we have to fix qemu-img create
>>>>>> to do not create out of spec geometry for images larger than 127G.
>>>>>> It should set the correct footer->size and then calculate the geometry.
>>>>> Do I understand correctly that you just volunteered to fix up that whole
>>>>> thing? ;-)
>>>> I knew that this would happen ;-)
>>>>
>>>> Regarding the C/H/S calculation. I was just wondering if we should
>>>> not set this to maximum (=invalid?) for all newly created images.
>>>> That is what disk2vhd does.
>>> CHS is what Virtual PC relies on. So I guess if you did that, you
>>> would render images unusable by it. Are you sure that disk2vhd does this
>>> always? I would have thought that it only does it for large images.
>> At least 2.0.1 (latest available version) does this as well as the version
>> that I used when I added the hack for d2v creator.
>>
>> Virtual PC would not be able to use images we create with qemu-img create
>> if we use footer->size (which I suppose to reanme to footer->cur_size, btw)
>> to calculate bs->total_sectors because we might write data to the end of
>> the image which gets truncated in CHS format.
> These kinds of problems are why I'd like to keep CHS and size always
> consistent when creating an image with qemu-img.

Okay, then I would vote for your RFC patch + fixing qemu-img create
to not generate out of spec CHS values and just set maximum which
then would make vpc_open use footer->size.

Peter


>
> Kevin


-- 

Mit freundlichen Grüßen

Peter Lieven

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