[libvirt] Re: [Qemu-devel] [PATCH 1/6] Allow multiple monitor devices (v2)
jamie at shareable.org
Tue Apr 14 18:19:08 UTC 2009
Avi Kivity wrote:
> Daniel P. Berrange wrote:
> >Yes indeed its a little crazy :-) As anthony mentioned if libvirt were
> >able to be notified of changes a user makes in the monitor, there's no
> >reason we could not allow end users to access the monitor of a VM
> >libvirt is managing. We just need to make sure libvirt doesn't miss
> >changes like attaching or detaching block devices, etc, because that'll
> >cause crash/data loss later when libvirt migrates or does save/restore,
> >etc because it'll launch QEMU with wrong args
> You still have an inherent race here.
> user: plug in disk
> libvirt: start migration, still without disk
> qemu: libvirt, a disk has been plugged in.
Then fix it. The race is not necessary.
user: plug in a disk
libvirt: lock VM against user changes incompatible with migration
qemu: libvirt, lock granted
libvirt: query for current disk state
libvirt: start migration, knows about the disk
The "libvirt, a disk has been plugged in" will be delivered but it's
not important. libvirt queries the state of things after it acquires
the lock and before it starts migration.
> That means that to debug a problem in the field you have to locate a
> guest's host, and follow it around as it migrates (or disable migration).
That's right you do. Is there any way to debug a guest without
disabling migration? I don't think there is at present, so of course
you have to disable migration when you debug. Another reason for that
"lock against migration" mentioned above.
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