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

[Marcelo Tosatti]

On Tue, Jul 05, 2011 at 04:37:08PM +0100, Stefan Hajnoczi wrote:
> On Tue, Jul 5, 2011 at 3:32 PM, Marcelo Tosatti <address@hidden> 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<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.
> >
> > 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
> >
> >> >
> >> >>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.
> 
> One use case I have in mind is the Live Backup approach that Jagane
> has been developing.  Here the backup solution only creates a snapshot
> for the period of time needed to read out the dirty blocks.  Then the
> snapshot is deleted again and probably contains very little new data
> relative to the base image.  The backup solution does this operation
> every day.
> 
> This is the pathalogical case for any approach that copies the entire
> base into a new file.  We could have avoided a lot of I/O by doing an
> in-place update.
> 
> I want to make sure this works well.

This use case does not fit the streaming scheme that has come up. Its a
completly different operation.

IMO it should be implemented separately.

> Stefan