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
