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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack
Date: Mon, 21 Jun 2010 16:22:59 +0100
References: <m3ocf8ts1y.fsf@blackfin.pond.sub.org>
	<4C1F7C6B.3040602@codemonkey.ws> <20100621150058.GA14072@lst.de>
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To: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Christoph Hellwig <chellwig@redhat.com>, Markus Armbruster <armbru@redhat.com>, Luiz Capitulino <lcapitulino@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, Christoph Hellwig <hch@lst.de>, Avi Kivity <avi@redhat.com>

> The user basically can specify two things:
> 
>  - a transport protocol.  Normally this is just the filesystem
>    interface, but it can also be nbd, http or for really sick people
>    vvfat.  This is a setting which can't be guessed, btw - it needs
>    to be explicitly set in some way, with file used as a reasonable
>    fallback.
>
>  - an image format.  This one interprets the content the transport
>    protocol delivers to us.  This can either be raw for not interpreting
>    it all, or things like qcow2 / vmdk to add more functionality to it.

Makes sense to me.

In theory we should be able to stack image formats. In practice it's probably 
not very useful with the current set of formats, so may not be worth the 
hassle. The best way to implement it may be to add a "blockdev" transport.

Paul