Re: [Qemu-devel] [PATCH 3/4] block/vpc: give option to force the current_size field in .bdrv_create

[Jeff Cody <jcody@redhat.com>]

On Wed, Feb 24, 2016 at 02:07:18PM +0100, Kevin Wolf wrote:
> Am 24.02.2016 um 13:44 hat Peter Lieven geschrieben:
> > if the size is forced I would set the chs values to max. this way no
> > new creator String is needed and it is even backwards compatible. this
> > is what disk2vhd does.
> 
> Does disk2vhd do it this way even if the size is smaller than the
> maximum that can be represented with CHS?
>

I don't know about disk2vhd, but I just created a 5G dynamic VHD
image on Hyper-V, and it produced:

cyl: 10402, heads: 16, secs: 63
virtual size: 5.0G (5368709120 bytes)

(the virtual size as calculated by CHS in that case would have been
5368430592 bytes)

I then tested the reverse - I modified qemu to create a VHD image with
5G as the current_size, but maxed out CHS parameters.  I imported it
into Hyper-V, and it worked fine - just recognized as a 5G disk with
5368709120 bytes.

But with all that, it seems like it may be better to mimic the Hyper-V
behavior, and use a new creator app string, with the normal CHS
values.


> > > Am 24.02.2016 um 13:24 schrieb Jeff Cody <jcody@redhat.com>:
> > > 
> > >> On Wed, Feb 24, 2016 at 11:19:37AM +0100, Kevin Wolf wrote:
> > >> Am 24.02.2016 um 01:47 hat Jeff Cody geschrieben:
> > >>> When QEMU creates a VHD image, it goes by the original spec,
> > >>> calculating the current_size based on the nearest CHS geometry (with an
> > >>> exception for disks > 127GB).
> > >>> 
> > >>> Apparently, Azure will only allow images that are sized to the nearest
> > >>> MB, and the current_size as calculated from CHS cannot guarantee that.
> > >>> 
> > >>> Allow QEMU to create images similar to how Hyper-V creates images, by
> > >>> setting current_size to the specified virtual disk size.  This
> > >>> introduces an option, force_size, to be passed to the vpc format during
> > >>> image creation, e.g.:
> > >>> 
> > >>>    qemu-img convert -f raw -o force_size -O vpc test.img test.vhd
> > >>> 
> > >>> Bug reference: https://bugs.launchpad.net/qemu/+bug/1490611
> > >>> 
> > >>> Signed-off-by: Jeff Cody <jcody@redhat.com>
> > >> 
> > >> We need to set a different creator string here that makes vpc_open()
> > >> recognise the image as current_size based.
> > >> 
> > >> Kevin
> > > 
> > > How about "qem2"?  I initially thought about just changing the case on
> > > "qemu", but I was afraid some other software may treat the app creator
> > > string as case-insensitive.
> > > 
> > > I'll also update patch 1, to recognize that string as well.
> 
> I had the same thoughts about our options, and I wasn't fully convinced
> of either, so I didn't propose any. I was leaning towards the case
> change, though, as I don't think it should make a difference and it
> reads nicer. But "qem2" is okay with me, too.
> 
> Kevin