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

[Dor Laor <dlaor@redhat.com>]

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)
   - 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.

>
> Compare that to image streaming, which works across crash and doesn't
> duplicate I/O.
>
> Stefan
>