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[Qemu-devel] Re: [kvm-devel] Storing command line options in qcow2 image

From: Avi Kivity
Subject: [Qemu-devel] Re: [kvm-devel] Storing command line options in qcow2 images
Date: Mon, 30 Jul 2007 11:36:19 +0300
User-agent: Thunderbird (X11/20070419)

Jorge Lucángeli Obes wrote:
Hi Avi, hi all,

I believe that Laurent (copied) is also interested in this area.

I've started some (very minor) groundwork for this task. My idea was
to add an extra "annotation" field in qcow2 snapshots. In this way, a
snapshot can hold abitrary information; for example, command line

Before going any further, I wanted to validate the general idea with
the list.

I've copied qemu-devel as well. This can benefit qemu as much as kvm and I see no reasons that this shouldn't be merged into kvm through qemu upstream. Furthermore, I'd very much hate to see qemu and kvm image formats diverge, so I'd like to get the qemu maintainer's approval for any format changes or extensions.

I am planning on using a snapshot with a distinct name to
store command line options. This approach seems simple yet it is
powerful enough for what we need to do.

Yes, that seems to be the best idea. A snapshot name perhaps beginning in a '\0' would avoid any ambiguity (much like the separate unix domain socket namespace).

There's a slight overhead as now all qcow2 snapshots have to store a
extra (probably) empty pointer. An alternative approach would be to
somehow indicate that some snapshots are "descriptive" snapshots and
others are "useful" snapshots; however, I felt that this would be more

Will storing an extra pointer break existing qcow2 images? If so, we'd better avoid it.

I have a working patch to qemu-img that allows storing arbitrary
annotations into qcow2 images. I developed it as a testing utility.
Right now annotations are implemented as another string alongside the
snapshot's 'name' and 'id'. I thought about storing annotations as
key/value pairs but again, it seemed overkill.

>From here, the plan would be to add a command to qemu-img that allows
embedding of command line options into qcow2 images. These options
would be stored in a snapshot with a special name, and qemu would
check for the presence of this snapshot in order to read command line
options from the qcow2 image.

Ideas? Suggestions? I can send the patches for the functionality
that's implemented right now.

error compiling committee.c: too many arguments to function

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