Re: [Qemu-devel] [PATCH 0/2] qemu block changes: keep backing file format v2

[Uri Lublin]

Jamie Lokier wrote:
> Uri Lublin wrote:
> > I call "hidden image format" to the following format of a backing file name:
> > "name\0format". Although it can be considered a hack, that's an easy
> > way to support old images with new qemu as well as old qemu with new images
> > (in which case probing would be done), without changing the qcow2 header.
> ...
> > Changes from v1: use "name\0format" instead of "fmt:FMT:name"
>
> Do you think there will be any other information added in future,
> while wanting to remain compatible with old qemus (i.e. without
> changing the qcow2 header)?
>
> If so, "name\0format=FMT", and stop at the second \0 if there is one,
> something like that?
>
> -- Jamie

I think you'd always want to keep the format of your backing file, so as long as
you keep the order of options fixed, it's safe (meaning we can add more options 
following the format, and there is no need to parse it).

If we are thinking about ways to extend the simple "name\0options" format,
Shahar Frank wrote a better, more extensible implementation, which saves image 
options as extensions following the qcow2-header. The options are saved in a 
binary format similar to the header. Each option has a little header of its own 
which includes magic(==type) and len.

We (Shahar) also implemented a zero-cluster (aka zero-dedup) and would like,
in the future, to keep the offset of that cluster.

If such an approach is acceptable, I can work on a patch implementing it (rebase 
and improve a bit Shahar's implementation).

Thanks,
    Uri.