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Re: [Qemu-block] [Qemu-devel] [PATCH v4 00/18] bitmaps: introduce 'bitma

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v4 00/18] bitmaps: introduce 'bitmap' sync mode
Date: Mon, 22 Jul 2019 13:21:02 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

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.)


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