[libvirt] [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration
Kevin Wolf
kwolf at redhat.com
Tue Sep 7 15:39:33 UTC 2010
Am 07.09.2010 17:30, schrieb Anthony Liguori:
> On 09/07/2010 10:20 AM, Kevin Wolf wrote:
>> Am 07.09.2010 17:11, schrieb Anthony Liguori:
>>> Copy-on-read is, in many cases, a property of the backing file because
>>> it suggests that the backing file is either very slow or potentially
>>> volatile.
>>>
>> The simple copy-on-read without actively streaming the rest of the image
>> is not enough anyway for volatile backing files.
>>
>
> But as a web site owner, it's extremely useful for me to associate
> copy-on-read with an image because it significantly reduces my bandwidth.
>
> I have a hard time believing this isn't a valuable use-case and not one
> that's actually pretty common.
As a web site user, I don't necessarily want you to control the
behaviour of my qemu. :-)
But I do see your point there.
>>> You really can't do as good of a job in the block layer because you have
>>> very little info about the characteristics of the disk image.
>>>
>> I'm not saying that the generic block layer should implement
>> copy-on-read. I just think that it should pass a run-time option to the
>> driver - maybe just a BDRV_O_COPY_ON_READ flag - instead of having the
>> information in the image file. From a user perspective it should look
>> the same for qed, qcow2 and whatever else (like copy-on-write today)
>>
>
> Okay, the only place I'm disagreeing slightly is that I think an image
> format should be able to request copy_on_read such that the default
> behavior if an explicit flag isn't specified is to do what the image
> suggests we do.
Maybe we can agree on that. I'm not completely decided yet if allowing
the image to contain such a hint is a good or a bad thing.
Kevin
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