Re: [Qemu-devel] [patch 6/7] QEMU live block copy

[Jagane Sundar <jagane@sundar.org>]

On 6/7/2011 5:15 AM, Stefan Hajnoczi wrote:
> On Mon, Jun 6, 2011 at 5:55 PM, Marcelo Tosatti<mtosatti@redhat.com>  wrote:
>
> I haven't reviewed this whole patch yet, but comments below.
>
> This patch, like image streaming, may hit deadlocks due to synchronous
> I/O emulation.  I discovered this problem when working on image
> streaming and it should be solved by getting rid of the asynchronous
> context concept.  The problem is that async I/O emulation will push a
> new context, preventing existing requests to complete until the
> current context is popped again.  If the image format has dependencies
> between requests (e.g. QED allocating writes are serialized), then
> this leads to deadlock because the new request cannot complete until
> the old one does, but the old one needs to wait for the context to be
> popped.  I think you are not affected by the QED allocating write case
> since the source image is only read, not written, by live block copy.
> But you might encounter this problem in other places.
>
Hello Stefan,

Can you expand on this some more? I have similar concerns for Livebackup.

At the beginning of your paragraph,  did you mean 'asynchronous I/O 
emulation' instead of 'synchronous I/O emulation'?

Also, I don't understand the 'stack' construct that you refer to. When 
you say 'push a new context', are you talking about what happens when a 
new thread picks up a new async I/O req from the VM, and then proceeds 
to execute the I/O req? What is this stack that you refer to?

Any design documents, code snippets that I can look, other pointers welcome.

Thanks,
Jagane