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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 0/2] qemu block changes: keep backing file format v2
Date: Tue, 27 Jan 2009 15:46:33 -0600
User-agent: Thunderbird (X11/20090105)

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?


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).


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