Re: [Qemu-devel] Image streaming and live block copy

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jun 17, 2011 at 10:36:21AM +0200, Kevin Wolf wrote:
> Am 16.06.2011 16:52, schrieb Marcelo Tosatti:
> > On Thu, Jun 16, 2011 at 03:08:30PM +0200, Kevin Wolf wrote:
> >> Am 16.06.2011 14:49, schrieb Avi Kivity:
> >>> On 06/16/2011 03:35 PM, Kevin Wolf wrote:
> >>>> * Image streaming is a normal image file plus copy-on-read plus a
> >>>> background task that copies data from the source image
> >>>
> >>> Or a block-mirror started in degraded mode.
> >>
> >> At least not in the same configuration as with live block copy: You
> >> don't want to write to the source, you only want to read from it when
> >> the destination doesn't have the data yet.
> >>
> >>>> * Live block copy is a block-mirror of two normal image files plus a
> >>>> background task that copies data from the source image
> >>>
> >>> = block-mirror started in degraded mode
> >>
> >>>> The right solution is probably to implement COR and the background task
> >>>> in generic block layer code (no reason to restrict it to QED) and use it
> >>>> for both image streaming and live block copy. (This is a bit more
> >>>> complicated than it may sound here because guest writes must always take
> >>>> precedence over a copy - but doing complicated things is an even better
> >>>> reason to do it in a common place instead of duplicating)
> >>>
> >>> Or in a block-mirror block format driver - generic code need not be 
> >>> involved.
> >>
> >> Might be an option. In this case generic code is only involved with the
> >> stacking of BlockDriverStates, which is already implemented (but
> >> requires -blockdev for a sane way to configure things).
> >>
> >> Kevin
> > 
> > What are the disadvantages of such an approach for image streaming,
> > versus the current QED approach?
> > 
> > blkstream block driver:
> > 
> > - Maintain in memory whether given block is allocated in local image,
> > if not, read from remote, write to local. Set block as local.
> > Local and remote simply two block drivers from image streaming driver
> > POV.
> 
> Why maintain it in memory? We already have mechanisms to track this in
> COW image formats, so that you can even continue after a crash.
> 
> We can still add a raw-cow driver that maintains the COW data in memory
> for allowing raw copies, if this is needed.

Well, then image streaming is not for generic-format anymore. OK, the
uptodate information can live in disk if supported by the lower level
format.

> > - Once all blocks are local, notify mgmt so it can switch to local
> > copy.
> > - Writes are mirrored to source and destination, minding guest writes
> > over copy writes.
> 
> Image streaming shouldn't write to the source. But adding a flag for
> this isn't a major problem.

OK, block copy does write to the source.

> > Over this scheme, you'd have:
> > 
> > 1) Block copy. 
> > Reopen image to be copied with
> > blkstream:/path/to/current-image:/path/to/destination-image,
> > background read sectors 0...N.
> > 
> > 2) Image stream:
> > blkstream:remote-image:/path/to/local-image,
> > background read sectors 0...N.
> > 
> > Where remote-image is remote accessible image such as NBD.
> 
> I think that should work.
> 
> By the way, we'll get problems with the colon syntax. Without -blockdev
> we'll have to invent a new syntax, maybe with brackets:
> 
> blkstream:[nbd:localhost]:out.qcow2
> 
> Kevin