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Date: Tue, 22 Jun 2010 17:46:06 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack
Message-ID: <20100622164606.GE4371@shareable.org>
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To: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Christoph Hellwig <chellwig@redhat.com>, qemu-devel@nongnu.org, Luiz Capitulino <lcapitulino@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, Avi Kivity <avi@redhat.com>

Markus Armbruster wrote:
> A possible reason why we currently expose format and protocol at the
> user interface is to avoid stacking there.

Pragmatic solution?: A few generic flags in each stacking module
("format/protocol/transport"), which govern which other modules are
allowed to stack on top or underneath.

For example, vvfat may provide a blockdev-like abstraction, along with
flags STACK_ABOVE_ONLY_RAW | STACK_BELOW_ONLY_DIRECTORY, which means
"raw" and "blkdebug" are allowed above (of course ;-) but other things
like the image formats shouldn't be.  And below, it can't stack on a
blockdev-like abstraction, but needs a directory and uses filesystem
operations on it - the thing that Plan9fs needs.

Btw, I think we expose "format" because "virtual disk image file
format" is a useful and meaningful concept to users.  When someone
needs to use a .VMDK file, they know it as a "VMDK format file", not
"I must use the VMDK protocol with this file".

-- Jamie