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

From: Supriya Kannery
Subject: Re: [Qemu-devel] CoW image commit+shrink(= make_empty) support
Date: Wed, 13 Jun 2012 16:26:15 +0530
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20110817 Fedora/3.1.12-1.fc14 Thunderbird/3.1.12

On 06/12/2012 04:26 PM, Stefan Hajnoczi wrote:
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

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

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

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

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.



  Sorry! for not following up these discussions and hence the delay in
responding. After few month's gap, I had resumed working on bdrv_reopen() patchset but was not aware of this dependency and urgency. Will be posting whatever I have by tomorrow so that Jeff can use that code as needed for block-commit.

- Supriya

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