[Qemu-devel] Re: [RFC PATCH] qed: add support for Copy-on-Read

[Stefan Hajnoczi]

On Wed, Mar 30, 2011 at 08:08:34PM -0500, Anthony Liguori wrote:
> When creating an image using qemu-img, just pass '-o copy_on_read' and then
> whenever QED reads from a backing file, it will write the block to the QED
> file after the read completes ensuring that you only fetch from the backing
> device once.
> 
> This is very useful for streaming images over a slow connection.
> 
> This isn't ready for merge yet as it's not playing nice with synchronize I/O.

What is the issue here?  Streaming had issues with aio contexts and
synchronous I/O emulation but copy-on-read by itself looks safe to me.

> I think it's fairly easy to do the same thing in qcow2 by just hooking adding
> some logic after bdrv_aio_write() to call back into qcow2 with a synchronous
> I/O write in the backing file case.   Thoughts on whether that would actually
> work?

Why not do the follow-up .bdrv_aio_writev() for qcow2 too?  I don't see
a reason to do it synchronously.

Stefan