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

From: Uri Lublin
Subject: Re: [Qemu-devel] qemu: block.c: introducing "fmt:FMT:" prefix to image-filenames
Date: Thu, 08 Jan 2009 21:09:08 +0200
User-agent: Thunderbird (X11/20081119)

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.

force backing file format to raw (no probing)
$ qemu-img create -f raw /tmp/image1.raw 10G
$ qemu-img create -b fmt:raw:/tmp/image1.raw -f qcow2 /tmp/image1.qcow2

  $ qemu-img create -F raw -b /tmp/image1.raw -f qcow2 /tmp/image1.qcow2

Or fat
$ qemu-system-x86_64 -hda fmt:qcow2:/tmp/uuu.qcow2 -hdb fat:floppy:/tmp/images

This is unneccessary, since -hda is deprecated, and there's a new -drive arg that already has ability to set format explicitly,
as well as many other flags that you need when setting up disks.

$ qemu-system-x86_64 \ -drive index=0,format=qcow2,file=/tmp/uuu.qcow2 \
         -drive index=1,format=fat:floppy,file=/tmp/images

Again that's true only for the leaf (writeable) images.

Maybe the examples I've shown were confusing.
The main issues we are trying to solve with this format are:
1. When opening a backing file, know what the format is to prevent probing.
2. Enable using qcow2 over host-devices (e.g qcow2 format image on an LVM lv such as /dev/mapper/mygroup-myvm)


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