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Re: [Qemu-devel] qemu: block.c: introducing "fmt:FMT:" prefix to image-f

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] qemu: block.c: introducing "fmt:FMT:" prefix to image-filenames
Date: Thu, 8 Jan 2009 19:13:06 +0000
User-agent: Mutt/1.4.1i

On Thu, Jan 08, 2009 at 09:09:08PM +0200, Uri Lublin wrote:
> Daniel P. Berrange wrote:
> >On Thu, Jan 08, 2009 at 08:15:25PM +0200, Uri Lublin wrote:
> >>The purpose of this prefix is to
> >>1. Provide a way to know the backing file format without probing
> >>   it (setting the format upon creation time).
> >>2. Enable using qcow2 format (and others) over host block devices.
> >>   (only if the user specifically asks for it).
> >>
> >>If no fmt:FMT: is provided we go back to probing.
> >
> >I still don't like the fact that this is inventing a second syntax
> >for specifying format that's different to the syntax used for  the 
> >existing -drive parameter, which is
> >
> >  -drive file=/some/path,format=qcow2,....other disk options...
> >
> >>backing file format is qcow2 (even though it's on a host block device)
> >>$ qemu-img create -b fmt:qcow2:/dev/loop0 -f qcow2 /tmp/uuu.qcow2
> >
> >I'd prefer to see a '-F' flag to specify format of backing file and
> >leave syntax of existing -b arg alone
> >
> > $ qemu-img create -F qcow2 -b /dev/loop0 -f qcow2 /tmp/uuu.qcow2
> I can easily modify qemu-img to support -F.
> The question is how to represent it such that when we run qemu it "knows" 
> the backing file format. In other words what will '-F qcow2' do ? In this 
> solution it will add 'fmt:qcow2:' as a prefix to the backing-file filename.

Yes the format you write into the qcow2 header can still use the 
fmt:qcow2: prefix - just no need to expose that particular qcow2
specific implementation detail on the CLI. 

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