Re: [Qemu-devel] KVM call agenda for June 28

[Kevin Wolf <kwolf@redhat.com>]

Am 05.07.2011 16:32, schrieb Marcelo Tosatti:
> On Tue, Jul 05, 2011 at 04:39:06PM +0300, Dor Laor wrote:
>> On 07/05/2011 03:58 PM, Marcelo Tosatti wrote:
>>> On Tue, Jul 05, 2011 at 01:40:08PM +0100, Stefan Hajnoczi wrote:
>>>> On Tue, Jul 5, 2011 at 9:01 AM, Dor Laor<dlaor@redhat.com>  wrote:
>>>>> I tried to re-arrange all of the requirements and use cases using this wiki
>>>>> page: http://wiki.qemu.org/Features/LiveBlockMigration
>>>>>
>>>>> It would be the best to agree upon the most interesting use cases (while we
>>>>> make sure we cover future ones) and agree to them.
>>>>> The next step is to set the interface for all the various verbs since the
>>>>> implementation seems to be converging.
>>>>
>>>> Live block copy was supposed to support snapshot merge.  I think the
>>>> current favored approach is to make the source image a backing file to
>>>> the destination image and essentially do image streaming.
>>>>
>>>> Using this mechanism for snapshot merge is tricky.  The COW file
>>>> already uses the read-only snapshot base image.  So now we cannot
>>>> trivally copy the COW file contents back into the snapshot base image
>>>> using live block copy.
>>>
>>> It never did. Live copy creates a new image were both snapshot and
>>> "current" are copied to.
>>>
>>> This is similar with image streaming.
>>
>> Not sure I realize what's bad to do in-place merge:
>>
>> Let's suppose we have this COW chain:
>>
>>   base <-- s1 <-- s2
>>
>> Now a live snapshot is created over s2, s2 becomes RO and s3 is RW:
>>
>>   base <-- s1 <-- s2 <-- s3
>>
>> Now we've done with s2 (post backup) and like to merge s3 into s2.
>>
>> With your approach we use live copy of s3 into newSnap:
>>
>>   base <-- s1 <-- s2 <-- s3
>>   base <-- s1 <-- newSnap
>>
>> When it is over s2 and s3 can be erased.
>> The down side is the IOs for copying s2 data and the temporary
>> storage. I guess temp storage is cheap but excessive IO are
>> expensive.
>>
>> My approach was to collapse s3 into s2 and erase s3 eventually:
>>
>> before: base <-- s1 <-- s2 <-- s3
>> after:  base <-- s1 <-- s2
>>
>> If we use live block copy using mirror driver it should be safe as
>> long as we keep the ordering of new writes into s3 during the
>> execution.
>> Even a failure in the the middle won't cause harm since the
>> management will keep using s3 until it gets success event.
> 
> Well, it is more complicated than simply streaming into a new
> image. I'm not entirely sure it is necessary. The common case is:
> 
> base -> sn-1 -> sn-2 -> ... -> sn-n
> 
> When n reaches a limit, you do:
> 
> base -> merge-1

Hm, I would expect that a case like this is important, too:

base <- sn-1 <- ... <- sn-n-1 <- sn-n <- ... <- sn-m

Which should be merged so that we get the following (i.e. deleting older
snapshots but retaining more recent ones):

base <- sn-merged <- sn-n <- ... <- sn-m

Kevin