Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API

[Vladimir Sementsov-Ogievskiy]

16.11.2017 00:20, John Snow wrote:
> On 11/13/2017 11:20 AM, Vladimir Sementsov-Ogievskiy wrote:
> > Hi all.
> >
> > There are three qmp commands, needed to implement external backup API.
> >
> > Using these three commands, client may do all needed bitmap management by
> > hand:
> >
> > on backup start we need to do a transaction:
> >   {disable old bitmap, create new bitmap}
> >
> > on backup success:
> >   drop old bitmap
> >
> > on backup fail:
> >   enable old bitmap
> >   merge new bitmap to old bitmap
> >   drop new bitmap
> Can you give me an example of how you expect these commands to be used,
> and why they're required?
>
> I'm a little weary about how error-prone these commands might be and the
> potential for incorrect usage seems... high. Why do we require them?

It is needed for incremental backup. It looks like bad idea to export 
abdicate/reclaim functionality, it is simpler
and clearer to allow user to merge/enable/disable bitmaps by hand.

usage is like this:

1. we have dirty bitmap bitmap0 for incremental backup.

2. prepare image fleecing (create temporary image with backing=our_disk)
3. in qmp transaction:
    - disable bitmap0
    - create bitmap1
    - start image fleecing (backup sync=none our_disk -> temp_disk)
4. export temp_disk (it looks like a snapshot) and bitmap0 through NBD

=== external tool can get bitmap0 and then copy some clusters through 
NBD ===

on successful backup external tool can drop bitmap0. or can save it and 
reuse somehow

on failed backup external tool can do the following:
    - enable bitmap0
    - merge bitmap1 to bitmap0
    - drop bitmap1

--
Best regards,
Vladimir