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

[Dor Laor]

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<address@hidden>  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.

> > 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
> >
> > 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