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Re: [Qemu-devel] CoW image commit+shrink(= make_empty) support

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] CoW image commit+shrink(= make_empty) support
Date: Tue, 12 Jun 2012 11:56:47 +0100

On Mon, Jun 11, 2012 at 4:37 PM, Jeff Cody <address@hidden> wrote:
> On 06/11/2012 10:24 AM, Stefan Hajnoczi wrote:
>> On Mon, Jun 11, 2012 at 1:50 PM, Kevin Wolf <address@hidden> wrote:
>>> Am 11.06.2012 14:09, schrieb Stefan Hajnoczi:
>>>> On Fri, Jun 8, 2012 at 6:46 PM, Jeff Cody <address@hidden> wrote:
>>>>> On 06/08/2012 12:11 PM, Kevin Wolf wrote:
>>>>>> Am 08.06.2012 16:32, schrieb Jeff Cody:
>>>>>>> On 06/08/2012 09:53 AM, Stefan Hajnoczi wrote:
>>>>>>>> On Fri, Jun 8, 2012 at 2:19 PM, Jeff Cody <address@hidden> wrote:
>>>>>>>>> On 06/08/2012 08:42 AM, Stefan Hajnoczi wrote:
>>>>>>>>>> Let's figure out how to specify block-commit so we're all happy, that
>>>>>>>>>> way we can avoid duplicating work.  Any comments on my notes above?
>>>>>>>>> I think we are almost completely on the same page - devil is in the
>>>>>>>>> details, of course (for instance, on how to convert the destination 
>>>>>>>>> base
>>>>>>>>> from r/o to r/w).
>>>>>>>> Great.  The atomic r/o -> r/w transition and the commit coroutine can
>>>>>>>> be implemented on in parallel.  Are you happy to implement the atomic
>>>>>>>> r/o -> r/w transition since you wrote bdrv_append()?  Then Zhi Hui can
>>>>>>>> assume that part already works and focus on implementing the commit
>>>>>>>> coroutine in the meantime.  I'm just suggesting a way to split up the
>>>>>>>> work, please let me know if you think this is good.
>>>>>>> I am happy to do it that way.  I'll shift my focus to the atomic image
>>>>>>> reopen in r/w mode.  I'll go ahead and post my diagrams and other info
>>>>>>> for block-commit on the wiki, because I don't think it conflicts with we
>>>>>>> discussed above (although I will modify my diagrams to not show commit
>>>>>>> from the top-level image).  Of course, feel free to change it as
>>>>>>> necessary.
>>>>>> I may have mentioned it before, but just in case: I think Supriya's
>>>>>> bdrv_reopen() patches are a good starting point. I don't know why they
>>>>>> were never completed, but I think we all agreed on the general design,
>>>>>> so it should be possible to pick that up.
>>>>>> Though if you have already started with your own work on it, Jeff, I
>>>>>> expect that it won't be much different because it's basically the same
>>>>>> transactional approach that you know and that we already used for group
>>>>>> snapshots.
>>>>> I will definitely use parts of Supriya's as it makes sense - what I
>>>>> started work on is similar to bdrv_append() and the current transaction
>>>>> approach, so there will be plenty in common to reuse, even with some
>>>>> differences.
>>>> I have CCed Supriya who has been working on the reopen patch series.
>>>> She is close to posting a new version.
>>> It's just a bit disappointing that it takes several months between each
>>> two versions of the patch series. We'd like to have this in qemu 1.2,
>>> not only in qemu 1.14.
>>> I can understand if someone can't find the time, but then allow at least
>>> someone else to pick it up.
>> Hey, don't shoot the messenger :).  I just wanted add Supriya to CC so
>> she can join the discussion and see how much overlap there is with
>> what she's doing.  We all contribute to QEMU from different angles and
>> with different priorities.  If there is a time critical dependency on
>> the reopen code then discuss it here and the result will be that
>> someone officially drives the feature on.
> I am more than happy to take the previous reopen() patches, and drive
> those forward, and also do whatever else is needed for live block
> commit.

Can you share with us whether you have enough time to complete the
reopen() patches you've been working on?  This functionality is a
dependency for the new block-commit command.  Jeff is willing to take
on the reopen() work if you do not have time.  Please let us know.


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