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

[Kevin Wolf]

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:
>>>> Note that block-commit cannot work on the top-most image since the
>>>> guest is still writing to that image and we might never be able to
>>>> copy all the data into the base image (the guest could write new data
>>>> as quickly as we copy it to the base).  The command should check for
>>>> this and reject the top-most image.
>>>
>>> By this you mean that you would like to disallow committing the
>>> top-level image to the base?  Perhaps there is a way to attempt to
>>> converge, and adaptively give more time to the co-routine if we are able
>>> to detect divergence.  This may require violating the 'speed' parameter,
>>> however, and make the commit less 'live'.
>>
>> Yes, I think we should disallow merging down the topmost image.  The
>> convergence problem is the same issue that live migration has.  It's a
>> hard problem and not something that is essential for block-commit.  It
>> would add complexity into the block-commit implementation - the only
>> benefit would be that you can merge down the last COW snapshot.  We
>> already have an implementation that does this: the "commit" command
>> which stops the guest :).
> 
> OK.  And, it is better to get this implemented now, and if desired, I
> suppose we can always revisit the convergence at a later date (or not at
> all, if there is no pressing case for it).

Not allowing live commit for the topmost image may be a good way to
break the problem down into more manageable pieces, but eventually the
feature isn't complete until you can do it.

Basically what you need there is the equivalent of an active mirror. I
wouldn't be surprised if actually code could be reused for both.

>>>> 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.

Kevin