Re: [Qemu-devel] PATCH: Control over drive open modes for backing file

[Jamie Lokier <jamie@shareable.org>]

Anthony Liguori wrote:
> So while I think it's valid to have a "read-only disk" exposed to the 
> guest, I don't think the user should have anything to do with how we 
> open the file.

All good points.

My concern (and it may be different to others) is when I branch an
image using a qcow2 with backing qcow (perhaps more than two deep).

The backing image is sometimes shared with other branches.

E.g. I might have a full install of an OS, and then have a branch
where I install software package A, and another branch with software
package B.  Both share the same base image.

In these, it's very important that I don't accidentally modify the
base image.  E.g. if I foolishly entered the 'commit' monitor command
in VM A, I'd *corrupt* the disk of VM B.

So it would be nice to open the base image read-only.

(Microsoft have a similar description in their documentation on 'disk
image differencing for maintaining test systems' for Virtual PC).

Now savevm snapshots (as opposed to -snapshot) confuse this picture.

Can I arbitrarily branch off different snapshots within a single qcow2
file?  Can I delete the 'base' snapshots safely?  Most important: what
happens if I have snapshots in a qcow2 file which has a base file, and
I type 'commit'?  Does it corrupt all the snapshots, effectively?

I find the snapshot facility a bit unclear as to what _exactly_ it
does to be honest, and am therefore wary of it.

Thanks,
-- Jamie