Re: [Qemu-devel] Debugging Migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* John Snow (jsnow@redhat.com) wrote:
> Is there a convenient way of "pausing" or stalling a live migration to
> allow methodical testing of race conditions?
> 
> I'd like to instrument something along the lines of:
> 
> (1) Live migration begins.
> (2) migration is artificially halted or paused, but QEMU is allowed to run.
> (3) Some additional qtest/QMP commands are received and processed.
> (4) migration is allowed to resume.
> 
> Does anyone have perhaps even test patches to instrument this sort of
> thing, or is it up to detective john to add it if he wants it?

If you catch it during the iterative stage you can probably just
gdb or ctrl-z the destination and the migration thread should block;
or alternatively migrate to a pipe and similarly ctrl-z what ever is
there.

Mostly I do a few things:
  1) use tracing to follow it
     mostly just stderr tracing, but I've done systemtap scripts
     for some hairy stuff.
  2) Set the migration speed (migrate_set_speed) very very low
  3) Keep the source busy dirtying memory.

Of course that does lead to the question of what fun problem are
you trying to debug?

Dave

> --js
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK