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

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [RFC] introduce a dynamic library to expose qemu block API
Date: Fri, 13 Jul 2012 10:16:40 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Jul 13, 2012 at 10:12:47AM +0100, Stefan Hajnoczi wrote:
> On Tue, Jul 10, 2012 at 09:18:01AM +0200, Paolo Bonzini wrote:
> > Il 10/07/2012 07:37, Wenchao Xia ha scritto:
> > >>
> > >> 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.
> > >>
> > >   Thank u for the idea. The .so is introduced to let program access the
> > > image more directly, parsing string is not so fast and it depends on
> > > another program's stdout output, I hope to get a faster way.
> > 
> > I doubt you actually have profiled it.
> I think speed is not the issue, instead it's just providing an API that
> external programs can use.  Management tools, backup software, custom
> administration tools, etc.  It's convenient to have an API.

Actually I think speed could well be quite relevant. In large deployments
it would not be surpising to see 1000's of images in a directory. If you
want to be able to query metadata about all of them at once, then being
able to open()+read(4k)+close() 1000 times is going to be dramatically
faster than doing fork()+execve(qemu-img) 1000 times.

NB, I still think qemu-img info should be able to return a JSON parsable
data format, regardless of what any block library does.

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