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

[Anthony Liguori <aliguori@us.ibm.com>]

On 04/01/2011 04:42 AM, Stefan Hajnoczi wrote:
> 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.

Here's the scenario that fails for me although I'm starting to suspect 
block/curl as the real culprit.

qemu-img create -f qed -o copy_on_read -b 
http://linux.nssl.noaa.gov/fedora/fedora/linux/releases/14/Fedora/x86_64/iso/Fedora-14-x86_64-DVD.iso 
cached_iso.img

qemu -cdrom cache_iso.img -boot d

And I have a patch that does a bunch of synchronous reads of the disk.

>> 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.

Oh, I assumed with coroutines that that would be the preference in 
qcow2.  If not, AIO is just as good for me :-)

Regards,

Anthony Liguori

> Stefan