Re: [Qemu-devel] [patch 6/7] QEMU live block copy

[Avi Kivity <avi@redhat.com>]

On 05/31/2011 07:06 PM, Marcelo Tosatti wrote:
> On Sun, May 29, 2011 at 11:54:25AM +0300, Avi Kivity wrote:
> >  On 05/24/2011 12:31 AM, Marcelo Tosatti wrote:
> >  >Support live image copy + switch. That is, copy an image backing
> >  >a guest hard disk to a destination image (destination image must
> >  >be created separately), and switch to this copy.
> >  >
> >  >Command syntax:
> >  >
> >  >block_copy device filename [-i] -- live block copy device to image
> >  >               -i for incremental copy (base image shared between src and destination)
> >  >
> >  >Please refer to qmp-commands diff for more details.
> >
> >  IMO it would have been nicer to use the mirror driver for all
> >  copying; there would be no STAGE_DIRTY; simply a background process
> >  that copies over all blocks, taking care not to conflict with
> >  ongoing writes.
>
> Don't see the advantage of doing that.

No dirty logging
No conflict with migration
Simpler conceptually (IMO)

> Disadvantages:
>
> - Guest write performance is affected during copying (guest writes
>    compete with stream of writes from copy).

Competes anyway with your background task?

> - Complexity to handle copy write conflict (there is no need to handle
>    that with current solution).

If new write from source A overlaps an in-flight write from source B, 
hold it in a list off the B request, and re-issue it on B completion.

> - Unability to have the mirror functionality in a separate driver.

Why?

> >  It would also remove the conflict with migration.
>
> There is no fundamental problem with migration, its simply unsupported
> now.

Why?

> >  But that's an implementation detail and can be changed later.
>


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