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Date: Thu, 08 Jan 2009 21:15:57 +0200
From: Uri Lublin <uril@redhat.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] qemu: block.c: introducing "fmt:FMT:" prefix to
	image-filenames
References: <496642BD.4010706@redhat.com> <20090108185256.GA8669@redhat.com>
	<49664F54.7050908@redhat.com> <20090108191301.GB8669@redhat.com>
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: qemu-devel@nongnu.org

Daniel P. Berrange wrote:
> 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. 

OK. I'll prepare a patch that adds '-F fmt' to 'qemu-img create -b ...'

Thanks,
    Uri.