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

[John Snow]

On 7/22/19 8:17 AM, Fabian Grünbichler wrote:
> 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.
> 

Not available quite yet; I added it in fairly hastily but haven't done
the testing I want to do yet, so I wouldn't feel comfortable sharing it
before I do my own due diligence on it. Give me a chance to polish it so
that the testing effort isn't wasted :)

Can you share some of your use-cases for how you are using the
"incremental mirror" so far? It might be useful for the patch
justification if I can point to production use cases. (And good for
allocating time, too.)

--js