Re: [Qemu-devel] Live migration + cpu/mem hotplug

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 10 Jan 2017 14:28:19 +0800
Bob Chen <a175818323@gmail.com> wrote:

> Answer my own question:
> 
> The corresponding cmd-line parameter for memory hot-add by QEMU monitor is,
> -object memory-backend-ram,id=mem0,size=1024M -device
> pc-dimm,id=dimm0,memdev=mem0
pc-dimm should have additional property
  -device pc-dimm,id=dimm0,memdev=mem0,hotplugged=on

> 
> 2017-01-05 18:12 GMT+08:00 Daniel P. Berrange <berrange@redhat.com>:
> 
> > On Thu, Jan 05, 2017 at 04:27:26PM +0800, Bob Chen wrote:  
> > > Hi,
> > >
> > > According to the docs, the destination Qemu must have the exactly same
> > > parameters as the source one. So if the source has just finished cpu or
> > > memory hotplug, what would the dest's parameters be like?
> > >
> > > Does DIMM device, or logically QOM object, have to be reflected on the  
> > new  
> > > command-line parameters?  
> >
> > Yes, if you have hotplugged any type of device since the VM was started,
> > the QEMU command line args on the target host must include all the original
> > args from the source QEMU, and also any args reflect to reflect the
> > hotplugged devices too.
> >
> > A further complication is that on the target, you must also make sure you
> > fully specify *all* device address information (PCI slots, SCSI luns, etc
> > etc), because the addresses QEMU assigns to a device after hotplug may
> > not be the same as the addresses QEMU assigns to a device whne coldplug.
> >
> > eg if you boot a guest with 1 NIC + 1 disk, and then hotplug a 2nd NIC
> > you might get
> >
> >    1st NIC  == PCI slot 2
> >    1st disk == PCI slot 3
> >    2nd NIC  == PCI slot 4
> >
> > if however, you started QEMU with 2 NICs and 1 disk straight away QEMU
> > might assign addresses in the order
> >
> >    1st NIC  == PCI slot 2
> >    2nd NIC  == PCI slot 3
> >    1st disk == PCI slot 4
> >
> > this would totally kill a guest OS during live migration as the slots
> > for devices its using would change.
> >
> > So as a general rule when launching QEMU on a target host for migrating,
> > you must be explicit about all device addresses and not rely on QEMU to
> > auto-assign addresses. This is quite alot of work to get right, but if
> > you're using libvirt it'll do pretty much all this automatically for
> > you.
> >
> > Regards,
> > Daniel
> > --
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> > :|
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> >