Re: [Qemu-devel] [RFC] introduce a dynamic library to expose qemu block API

[Daniel P. Berrange]

On Mon, Jul 09, 2012 at 04:54:08PM +0800, Wenchao Xia wrote:
> Hi, Paolo and folks,
>   qemu have good capabilities to access different virtual disks, I want
> to expose its block layer API to let 3rd party program linked in, such
> as management stack or block tools, to access images data directly.
> 
> Following is the objects:
>   (1) API to write/read block device at offset.
>   (2) Determine the image type,qcow2/qed/raw
>   (3) Determine which blocks are allocated.
>   (4) Determine backing file.
> 
> Following is my implementing plan draft:
>   1 introduce libqblock.so in sub directory in qemu.
>   2 write a nbd client in libqblock, similar to qemu nbd client. Then
> use it to talk with nbd server, by default is qemu-nbd, to get access
> to images. In this way, libqblock.so could be friendly LGPL licensed.
>   3 still not got a good way to get additional info in (2)(3)(4),
> currently in my head is patch qemu-nbd to add an additional nbd command,
> "image-info", in which returns related info.
> 
>   What do you think about it?

For arbirary read/write access to disk images, I can see a little value
in having a standalone libnbd client API, that is able to just talk to
any NBD server. Arguably such a thing does not need to be part of the
QEMU source tree - eg see the recently written libiscsi.so client.

For getting the other metadata about the disk image you mention, another
possibility to is just make 'qemu-img info' return the data in a machine
parseable format, ie JSON & make a client API for extracting data from
this JSON document.

For a full-blown RPC API for doing arbitrary tasks related to block
devices, then many apps will tend towards libguestfs, since it provides
such a wide range of functionality for manipulating disk images.

Regards,
Daniel
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