[Qemu-devel] backing file support for vpc/vhd ?

[Qemu-devel] backing file support for vpc/vhd ?

[Philipp Hahn]

Hello,

while working on a Xen-VM I noticed that qemu-img does not support the backing file mechanism of vpc/vhd files, but Xens blktap2 does; it has a libvhd and some tools to create and modify vpc/vhd files:

tools/blktap2/vhd/lib/atomicio.c: BSD (2 clause)
tools/blktap2/vhd/lib/vhd-util-check.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-modify.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-uuid.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-scan.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-revert.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-snapshot.c: BSD (3 clause)
tools/blktap2/vhd/lib/libvhd.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-query.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-set-field.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-repair.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-fill.c: BSD (3 clause)
tools/blktap2/vhd/lib/libvhd-journal.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-read.c: BSD (3 clause)
tools/blktap2/vhd/lib/relative-path.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-coalesce.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-create.c: BSD (3 clause)
tools/blktap2/vhd/lib/vhd-util-resize.c: BSD (3 clause)
tools/blktap2/vhd/vhd-update.c: BSD (3 clause)
tools/blktap2/vhd/vhd-util.c: BSD (3 clause)

Is the lack of support for backing files in vpc/vjd in qemu just a question of time / interest, are there technical problems, or are there some political issues?

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn@univention.de
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

Re: [Qemu-devel] backing file support for vpc/vhd ?

[Kevin Wolf]

Am 19.06.2013 um 16:02 hat Philipp Hahn geschrieben:
> Hello,
> 
> while working on a Xen-VM I noticed that qemu-img does not support the backing file mechanism of vpc/vhd files, but Xens blktap2 does; it has a libvhd and some tools to create and modify vpc/vhd files:
> [...]
> 
> Is the lack of support for backing files in vpc/vjd in qemu just a question of time / interest, are there technical problems, or are there some political issues?

Nobody wrote the code, that's all.

Read-only support should be almost trivial, and for read-write we'd have
to add some real code to deal with the block bitmaps (without backing
files we get away with what is mostly a slow fake implementation).

Kevin