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

[Dor Laor <dlaor@redhat.com>]

On 07/05/2011 05:32 PM, Marcelo Tosatti wrote:
> 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
>
> You're potentially copying similar amount of data when merging back into
> a single image (and you can't easily merge multiple snapshots).
>
> If the amount of data thats not in 'base' is large, you create
> leave a new external file around:
>
> base ->  merge-1 ->  sn-1 ->  sn-2 ... ->  sn-n
> to
> base ->  merge-1 ->  merge-2

Sometimes one will want to merge the snapshot immediately post the base 
was backed-up

>
>>>
>>>> It seems like snapshot merge will require dedicated code that reads
>>>> the allocated clusters from the COW file and writes them back into the
>>>> base image.
>>>>
>>>> A very inefficient alternative would be to create a third image, the
>>>> "merge" image file, which has the COW file as its backing file:
>>>> snapshot (base) ->   cow ->   merge
>
> Remember there is a 'base' before snapshot, you don't copy the entire
> image.

Not always, the image might be raw file/device -

1. raw image
2. live snapshot it and use COW above it
    raw <- s1
3. backup the raw image using 3rd party mechanism
4. live merge (copy) s1 into raw

>
>>>>
>>>> All data from snapshot and cow is copied into merge and then snapshot
>>>> and cow can be deleted.  But this approach is results in full data
>>>> copying and uses potentially 3x space if cow is close to the size of
>>>> snapshot.
>>>
>>> Management can set a higher limit on the size of data that is merged,
>>> and create a new base once exceeded. This avoids copying excessive
>>> amounts of data.
>>>
>>>> Any other ideas that reuse live block copy for snapshot merge?
>>>>
>>>> Stefan
>>>
>>>