Re: [Qemu-devel] [PATCH] migration: Don't activate block devices if using -S

[Kevin Wolf <kwolf@redhat.com>]

Am 09.04.2018 um 12:27 hat Dr. David Alan Gilbert geschrieben:
> * Kevin Wolf (kwolf@redhat.com) wrote:
> > Am 03.04.2018 um 22:52 hat Dr. David Alan Gilbert geschrieben:
> > > * Kevin Wolf (kwolf@redhat.com) wrote:
> > > > Am 28.03.2018 um 19:02 hat Dr. David Alan Gilbert (git) geschrieben:
> > > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > > > 
> > > > > Activating the block devices causes the locks to be taken on
> > > > > the backing file.  If we're running with -S and the destination libvirt
> > > > > hasn't started the destination with 'cont', it's expecting the locks are
> > > > > still untaken.
> > > > > 
> > > > > Don't activate the block devices if we're not going to autostart the VM;
> > > > > 'cont' already will do that anyway.
> > > > > 
> > > > > bz: https://bugzilla.redhat.com/show_bug.cgi?id=1560854
> > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > 
> > > > I'm not sure that this is a good idea. Going back to my old writeup of
> > > > the migration phases...
> > > > 
> > > > https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg07917.html
> > > > 
> > > > ...the phase between migration completion and 'cont' is described like
> > > > this:
> > > > 
> > > >     b) Migration converges:
> > > >        Both VMs are stopped (assuming -S is given on the destination,
> > > >        otherwise this phase is skipped), the destination is in control of
> > > >        the resources
> > > > 
> > > > This patch changes the definition of the phase so that neither side is
> > > > in control of the resources. We lose the phase where the destination is
> > > > in control, but the VM isn't running yet. This feels like a problem to
> > > > me.
> > > 
> > > But see Jiri's writeup on that bz;  libvirt is hitting the opposite
> > > problem;   in this corner case they can't have the destination taking
> > > control yet.
> > 
> > I wonder if they can't already grant the destination QEMU the necessary
> > permission in the pre-switchover phase. Just a thought, I don't know how
> > this works in detail, so it might not possible after all.
> 
> It's a fairly hairy failure case they had; if I remember correctly it's:
>   a) Start migration
>   b) Migration gets to completion point
>   c) Destination is still paused
>   d) Libvirt is restarted on the source
>   e) Since libvirt was restarted it fails the migration (and hence knows
>      the destination won't be started)
>   f) It now tries to resume the qemu on the source
> 
> (f) fails because (b) caused the locks to be taken on the destination;
> hence this patch stops doing that.  It's a case we don't really think
> about - i.e. that the migration has actually completed and all the data
> is on the destination, but libvirt decides for some other reason to
> abandon migration.

If you do remember correctly, that scenario doesn't feel tricky at all.
libvirt needs to quit the destination qemu, which will inactivate the
images on the destination and release the lock, and then it can continue
the source.

In fact, this is so straightforward that I wonder what else libvirt is
doing. Is the destination qemu only shut down after trying to continue
the source? That would be libvirt using the wrong order of steps.

> > > > Consider a case where the management tool keeps a mirror job with
> > > > sync=none running to expose all I/O requests to some external process.
> > > > It needs to shut down the old block job on the source in the
> > > > 'pre-switchover' state, and start a new block job on the destination
> > > > when the destination controls the images, but the VM doesn't run yet (so
> > > > that it doesn't miss an I/O request). This patch removes the migration
> > > > phase that the management tool needs to implement this correctly.
> > > > 
> > > > If we need a "neither side has control" phase, we might need to
> > > > introduce it in addition to the existing phases rather than replacing a
> > > > phase that is still needed in other cases.
> > > 
> > > This is yet another phase to be added.
> > > IMHO this needs the managment tool to explicitly take control in the
> > > case you're talking about.
> > 
> > What kind of mechanism do you have in mind there?
> > 
> > Maybe what could work would be separate QMP commands to inactivate (and
> > possibly for symmetry activate) all block nodes. Then the management
> > tool could use the pre-switchover phase to shut down its block jobs
> > etc., inactivate all block nodes, transfer its own locks and then call
> > migrate-continue.
> 
> Yes it was a 'block-activate' that I'd wondered about.  One complication
> is that if this now under the control of the management layer then we
> should stop asserting when the block devices aren't in the expected
> state and just cleanly fail the command instead.

Requiring an explicit 'block-activate' on the destination would be an
incompatible change, so you would have to introduce a new option for
that. 'block-inactivate' on the source feels a bit simpler.

And yes, you're probably right that we would have to be more careful to
catch inactive images without crashing. On the other hand, it would
become a state that is easier to test because it can be directly
influenced via QMP rather than being only a side effect of migration.

Kevin