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

[Anthony Liguori <anthony@codemonkey.ws>]

Uri Lublin wrote:
> 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,

How about just extending the qcow2 header instead of playing ugly games 
with the backing_file field?

Regards,

Anthony Liguori

> 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.
>
>
>