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Re: [Qemu-devel] [PATCH 0/2] qemu block changes: keep backing file forma
Re: [Qemu-devel] [PATCH 0/2] qemu block changes: keep backing file format v2
Wed, 28 Jan 2009 01:11:58 +0200
Thunderbird 18.104.22.168 (X11/20090105)
Anthony Liguori wrote:
Uri Lublin wrote:
Jamie Lokier wrote:
Uri Lublin wrote:
I call "hidden image format" to the following format of a backing
"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
(in which case probing would be done), without changing the qcow2
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?
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"
How about just extending the qcow2 header instead of playing ugly games
with the backing_file field?
Would that be considered qcow3 ?
Shahar's extensions are following in the qcow2 header (without breaking existing
images). I forgot to mention they are only implemented for qcow2 images.
Currently backing files are being opened in block.c:bdrv_open2(). If we want to
do something specific to qcow2 (with regards to the backing file format), we'll
need to leave bs->backing_file empty and open the backing file from
block-qcow2.c:qcow_open(). Or we can add a backing_format field to struct
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
We (Shahar) also implemented a zero-cluster (aka zero-dedup) and would
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).