Re: [Qemu-block] [PATCH] mirror: add target-zeroed flag

[Stefan Hajnoczi]

On Tue, Jun 07, 2016 at 07:30:19PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 03.06.2016 18:45, Denis V. Lunev wrote:
> > On 06/03/2016 06:06 PM, Eric Blake wrote:
> > > On 06/03/2016 08:05 AM, Vladimir Sementsov-Ogievskiy wrote:
> > > > Add target-zeroed flag to allow user specify that target is already
> > > > zeroed. With this flag set zeroes which was in source before mirror
> > > > start will not be copyed.
> > > With this flag set, any runs of zeroes in the source before the mirror
> > > starts will not be copied.
> > > 
> > > > Without this libvirt migration of empty disk takes too long time.
> > > > 
> > > > Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> > > > ---
> > > > 
> > > > I've tested it with
> > > > time virsh migrate --live test qemu+ssh://other_node/system
> > > > --copy-storage-all
> > > Presumably with a libvirt patch to turn on the optional flag.
> > > 
> > > I'm not sure I like this patch.  Libvirt uses NBD to implement
> > > --copy-storage-all, I think we're better off improving NBD to
> > > automatically handle sparse writes, than we are to add a one-off hack
> > > that requires libvirt to change.  That is, once NBD is smarter, the copy
> > > will be faster without needing a tweak.  And we ARE working on making
> > > NBD smarter (one of my goals for the 2.7 release is to get all the
> > > sparse file additions to NBD implemented)
> > > 
> > > That said, I'll still review it.
> > this is not enough, definitely.
> > 
> > There is a problem that mirror_iteration code sleeps even for
> > not read zeroes (and this IS slow). Moreover, even sending sparcified
> > zeroes takes a lot of time for round trips.
> > 
> > We have started with that and spent a lot of time trying to improve
> > the situation.
> > 
> > Also, as a side note, the QCOW2 file on a source and a target will
> > be different without the flag - original image has empty blocks,
> > target image will have blocks explicitly marked with zeroes.
> > 
> > Though this is a matter of taste... For us this approach is the simplest.
> > 
> > Den
> 
> Hey, what do think about it? Don't we forget somebody to be cc'ed?

Jeff Cody <address@hidden> maintains block jobs.  This patch should go
through him.

Stefan

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