Re: [Qemu-devel] [RFC PATCH 0/5] asynchronous migration state change handlers

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> Absolutely not.  This is hideously ugly and affects a bunch of code.
> 
> Spice is *not* getting a hook in migration where it gets to add
> arbitrary amounts of downtime to the migration traffic.  That's a
> terrible idea.
> 
> I'd like to be more constructive in my response, but you aren't
> explaining the problem well enough for me to offer an alternative
> solution.  You need to find another way to solve this problem.

Very short version:  The requirement is simply to not kill qemu on the
source side until the source spice-server has finished session handover
to the target spice-server.

Long version:  spice-client connects automatically to the target
machine, so the user ideally doesn't notice that his virtual machine was
just migrated over to another host.

Today this happens via "switch-host", which is a simple message asking
the spice client to connect to the new host.

We want move to "seamless migration" model where we don't start over
from scratch, but hand over the session from the source to the target.
Advantage is that various state cached in spice-client will stay valid
and doesn't need to be retransmitted.  It also requires a handshake
between spice-servers on source and target.  libvirt killing qemu on the
source host before the handshake is done isn't exactly helpful.

[ Side note: In theory this issue exists even today: in case the data
  pipe to the client is full spice-server will queue up the switch-host
  message and qemu might be killed before it is sent out.  In practice
  it doesn't happen though because it goes through the low-traffic main
  channel so the socket buffers usually have enougth space. ]

So, the big question is how to tackle the issue?

Option (1): Wait until spice-server is done before signaling completion
to libvirt.  This is what this patch series implements.

Advantage is that it is completely transparent for libvirt, thats why I
like it.

Disadvantage is that it indeed adds a small delay for the spice-server
handshake.  The target qemu doesn't process main loop events while the
incoming migration is running, and because of that the spice-server
handshake doesn't run in parallel with the final stage of vm migration,
which it could in theory.

BTW: There will be no "arbitrary amounts of downtime".  Seamless spice
client migration is pretty pointless if it doesn't finish within a
fraction of a second, so we can go with a very short timeout there.

Option (2): Add a new QMP event which is emmitted when spice-server is
done, then make libvirt wait for it before killing qemu.

Obvious disadvantage is that it requires libvirt changes.

Option (3): Your suggestion?

thanks,
  Gerd