Re: [Qemu-devel] Image streaming and live block copy

[Dor Laor <dlaor@redhat.com>]

On 06/24/2011 12:28 PM, Stefan Hajnoczi wrote:
> On Sun, Jun 19, 2011 at 5:02 PM, Dor Laor<dlaor@redhat.com>  wrote:
>> On 06/18/2011 12:17 PM, Stefan Hajnoczi wrote:
>>>
>>> On Sat, Jun 18, 2011 at 10:15 AM, Stefan Hajnoczi<stefanha@gmail.com>
>>>   wrote:
>>>>
>>>> On Fri, Jun 17, 2011 at 1:31 PM, Marcelo Tosatti<mtosatti@redhat.com>
>>>>   wrote:
>>>>>
>>>>> On Thu, Jun 16, 2011 at 04:30:18PM +0100, Stefan Hajnoczi wrote:
>>>>>>
>>>>>> On Thu, Jun 16, 2011 at 11:52:43AM -0300, Marcelo Tosatti wrote:
>>>>>> This approach does not use the backing file feature?
>>>>>>
>>>>>>> blkstream block driver:
>>>>>>>
>>>>>>> - Maintain in memory whether given block is allocated in local image,
>>>>>>> if not, read from remote, write to local. Set block as local.
>>>>>>> Local and remote simply two block drivers from image streaming driver
>>>>>>> POV.
>>>>>>> - Once all blocks are local, notify mgmt so it can switch to local
>>>>>>> copy.
>>>>>>> - Writes are mirrored to source and destination, minding guest writes
>>>>>>> over copy writes.
>>>>>>
>>>>>> We open the remote file read-only for image streaming and do not want
>>>>>> to
>>>>>> mirror writes.
>>>>>
>>>>> Why not? Is there any disadvantage of mirroring writes?
>>>>
>>>> Think of the use case with a Fedora master image over NFS.  You want a
>>>> local clone of that master image and use the stream command to copy
>>>> the data from the master image into the local clone.
>>>>
>>>> You cannot modify that master image because other VMs are using it too
>>>> and/or you want to be able to clone new VMs from it in the future.
>>>
>>> BTW the workaround is to create two local images:
>>> 1. Local clone with master image as a backing file.  This is the live
>>> block copy source image.
>>> 2. Local image without a backing file.  This is the live block copy
>>> destination image.
>>>
>>> But this is not very elegant.  Writes get mirrored so that crash recovery
>>> works.
>>
>> There is an easier work around for image streaming using live block copy
>> (mirror approach):
>>   - Create the dst VM as an empty new COW image of the src (even over
>>     the non shared storage, use some protocol tag for the src location
>>     like nbd://original_path/src_file_name)
>
> Migration and non-shared storage has come up a few times in this
> discussion.  But both live block copy and image streaming need access
> to source and destination - they do not have explicit non-shared
> storage support.  I think non-shared and using nbd:// is orthogonal to
> the discussion.  Just want to check that you agree and I haven't
> missed something?

You're right, I was mainly trying to be as general as possible.

>
>>   - Run the usual live block copy of src image (master read only OS
>>     template) to the destination.
>>     - Use a -src-read-only flag that will make the copy skip the src
>>       writing.
>>
>> Voila - no duplicate writes, crash recovery works since we reference the
>> original image and we share the code.
>
> So the running guest is using the destination image since the source
> is read-only?

Yes. The image will run on the destination not because of the source in 
RO state but because that is what we look for.

>
> This approach makes sense to me.
>
> Stefan
>
>
>