Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 21.06.2010 18:21, schrieb Markus Armbruster:
>> Christoph Hellwig <address@hidden> writes:
>> 
[...]
>>> 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.
>> 
>> You describe the special case where format and protocol make some sense:
>> you have a block driver that can transport bits in arbitrary formats,
>> and a block driver that interprets bits without caring for transport.
>
> No, that's not a special case. It's the only case there is.
>
>> In the general case, we have things like vvfat that make people wonder
>> whether it's a format or a protocol.  You can't stack it onto a
>> transport, so it can't be a format!  You can't stack a format on it, so
>> it can't be a protocol!
>
> Sure you can stack formats on it. raw and blkdebug are the ones that
> will be happy with the data that vvfat provides.

You can stack raw and blkdebug on *anything*.  And that includes qcow2.
Does that make qcow2 a protocol?  Are we running in circles yet?  :)

I've come the conclusion that distinguishing between format and protocol
doesn't help.  It only breeds confusion (and e-mail, lots of e-mail).
I've rethought blockdev_add, and I think I can do quite well without
format and protocol.  Let me try that in practice.  I'll post revised
docs & code when I have them.