Re: [Qemu-devel] [PATCH 2/4] block: add block topology options

[Jamie Lokier <jamie@shareable.org>]

Christoph Hellwig wrote:
> On Wed, Feb 03, 2010 at 01:00:47PM -0600, Anthony Liguori wrote:
> > But I don't think this is the wrong place to do it.  The 
> > BlockDriverState reflects that backing device, not the emulated device 
> > itself.  In this case, you're trying to set a property of the emulated 
> > device.
> 
> I think that's very borderline.  While the emulated device exposes these
> properties, they are in fact a property of the backing storage, the
> right sector and min/max I/O sizes are determined by the backing storage
> device.
> 
> > I think these need to be qdev properties of the respective devices.  
> > From a UI perspective, you can still expose -drive options for the end 
> > user to consume, but this data should be associated with the devices 
> > themselves.
> 
> In addition to not really beeing more logical this would be a lot more
> effort.  We'd need to add properties to all the device, which means
> including dealing with the n+1 ide variants, the virtio-pci proxy, etc.
> 
> If you believe it really needs to be in the qdev properties I'll
> implement it, but I suspect the current version is a better idea.

If you move your VM to a new system with different backing devices,
sometimes you want to be sure there is no guest-visible change.  Or
even if you just replace a drive - you might prefer confidence that
the guest sees no change.

Even if you just convert between qcow2 and a raw block device, or the
other way, you'll sometimes want to be sure it's not guest-visible.

-- Jamie