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Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack
From: |
Christoph Hellwig |
Subject: |
Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack |
Date: |
Mon, 28 Jun 2010 12:28:15 +0200 |
User-agent: |
Mutt/1.3.28i |
On Mon, Jun 21, 2010 at 06:21:09PM +0200, Markus Armbruster wrote:
> 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.
>
> 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!
Instead of starting to get hung up on the protocol name let's go back to
the basic problem. We have two types of Block drivers in qemu, which
are fundamentally different:
- leaf drivers which use some sort of native I/O method and present
an image. The typical cases for this are file/host_device/nbd/http/
ceph/sheepdog which just transport arbitrary content over some transport.
Another category makes up the content of the image on the fly. This
is generally an extreme hack as it's very hard to keep any kind of
coherency if the underlying content changes, but we've unfortunately
enough done it anyway for vvfat. I don't think I would accept any
addition driver of this type.
- non-leaf drivers which stack on top of another qemu block driver.
I think the difference is important enough for a user to care. It's
basically two different questions for the users:
- what image format do I want
- and where do I store that image
Even vvfat kinda fits into that model. The users wants to store the
image as a "life" view of a directory hierachy on the host. Because
of the limitation of that model the choice of image format ontop of
that storage solution is limited to raw.
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, (continued)
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Anthony Liguori, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Christoph Hellwig, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Paul Brook, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Anthony Liguori, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Christoph Hellwig, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Anthony Liguori, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Markus Armbruster, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Markus Armbruster, 2010/06/21
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Kevin Wolf, 2010/06/22
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Markus Armbruster, 2010/06/22
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack,
Christoph Hellwig <=
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Jamie Lokier, 2010/06/22
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Anthony Liguori, 2010/06/21
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Kevin Wolf, 2010/06/22
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Anthony Liguori, 2010/06/22
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Kevin Wolf, 2010/06/22
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Anthony Liguori, 2010/06/22
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Markus Armbruster, 2010/06/21
- [Qemu-devel] Re: block: format vs. protocol, and how they stack, Kevin Wolf, 2010/06/22
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Jamie Lokier, 2010/06/22
- Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack, Daniel P. Berrange, 2010/06/22