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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jul 05, 2011 at 06:04:34PM +0300, Dor Laor wrote:
> 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

Well, ok, this needs a separate interface for management, needs write
mirroring, and must mind crash handling.

> >>>>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
> >>>
> >>>