Re: [Qemu-devel] [PATCH v4 00/18] bitmaps: introduce 'bitmap' sync mode

[Fabian Grünbichler]

On Tue, Jul 09, 2019 at 07:25:32PM -0400, John Snow wrote:
> This series adds a new "BITMAP" sync mode that is meant to replace the
> existing "INCREMENTAL" sync mode.
> 
> This mode can have its behavior modified by issuing any of three bitmap sync
> modes, passed as arguments to the job.
> 
> The three bitmap sync modes are:
> - ON-SUCCESS: This is an alias for the old incremental mode. The bitmap is
>               conditionally synchronized based on the return code of the job
>               upon completion.
> - NEVER: This is, effectively, the differential backup mode. It never clears
>          the bitmap, as the name suggests.
> - ALWAYS: Here is the new, exciting thing. The bitmap is always synchronized,
>           even on failure. On success, this is identical to incremental, but
>           on failure it clears only the bits that were copied successfully.
>           This can be used to "resume" incremental backups from later points
>           in times.
> 
> I wrote this series by accident on my way to implement incremental mode
> for mirror, but this happened first -- the problem is that Mirror mode
> uses its existing modes in a very particular way; and this was the best
> way to add bitmap support into the mirror job properly.
> 
> [...]
> 
> Future work:
> [..]
>  - Add these modes to Mirror. (Done*, but needs tests.)

are these mirror patches available somehwere for testing in combination
with this series? your bitmaps branch does not seem to contain them ;)

we've been experimenting with Ma Haocong's patch (v4 from February) to add
"incremental"/differential sync to drive-mirror recently with positive
results so far, and this sounds like it is another attempt at getting
this properly integrated into Qemu.