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Re: [Qemu-block] [PATCH 3/4] block/vpc: give option to force the current

From: Jeff Cody
Subject: Re: [Qemu-block] [PATCH 3/4] block/vpc: give option to force the current_size field in .bdrv_create
Date: Wed, 24 Feb 2016 16:17:24 -0500
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Feb 24, 2016 at 08:28:20PM +0100, Peter Lieven wrote:
> Am 24.02.2016 um 14:40 schrieb Jeff Cody:
> > 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.
> So the idea to set CHS to MAX is not that bad.

I agree.  I just did a test with disk2vhd, against an empty (but
formatted) 7.5G usb drive.  Here is what it created:

# ./qemu-img info /mnt/nfs-2/tmp/WINtsts2.VHD
cyls: 65535, heads: 16, sectors: 255
image: /mnt/nfs-2/tmp/WINtsts2.VHD
file format: vpc
virtual size: 7.5G (8095006720 bytes)

I really like how one company has 3 different tools that all handle
this differently.

> >
> > 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.
> But this means that it is not backwards compatible. Maxing out CHS
> when forcing the size would mean all old qemu version would look
> at current size and not CHS.

That is a valid concern.  So, how about this:

With 'force_size' during image create, set the CHS parameters to MAX
(as d2v does), but also set the creator app to 'qem2'.  This at least
gives other software a chance to learn that new creator app and handle
it differently, if needed (and it shouldn't hurt anything, and will
still be backwards compatible).

> Might it be that Hyper-V and others use CHS when an ATA disk is emulated
> and the cur_size otherwise? I think this is what virtualbox does.

I tested the Hyper-V created image under a Linux VM in Hyper-V.  The
VM is using IDE controllers, and the image size was the current_size,
not the CHS calculation.


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