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

[Kevin Wolf]

Am 31.03.2011 03:08, schrieb Anthony Liguori:
> 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.

As you probably know, I don't agree with the interface. Copy on read
should be first and foremost a runtime option. It's okay to fetch the
default for this option from the image file, but it's not the right
primary interface.

> 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.

Like Stefan I'm not sure what you mean here.

One problem that I see is that if you have a concurrent write request
from the guest, the COR write request may overwrite the guest's request,
which is obviously wrong. Is this what you mean?

> 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?

I can't see a reason why it wouldn't work. Except that it has the same
problem that I described above. A synchronous write isn't going to solve
this, you'd additionally need a qemu_aio_flush() if you want to avoid
proper locking. But that's really the same for QED and qcow2.

Kevin