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Re: [libvirt] VN-Link vNIC memory state copying on VM Migration



Jagath,
  
> -----Original Message-----
> From: libvir-list-bounces redhat com [mailto:libvir-list-bounces redhat com] On Behalf Of Jagath
> Weerasinghe
> Sent: Friday, April 13, 2012 6:17 PM
> To: libvir-list redhat com
> Subject: Re: [libvirt] VN-Link vNIC memory state copying on VM Migration
> 
> Chris,
> 
> Thanks for the reply.
> 
> >  what memory state do you refer to exactly?
> What I meant by the memory state is the traffic being
> transferred through the vNIC when VM migration is triggered.

Since you are worried about the traffic I assume we are talking about
_live_ migration.

Libvirt qemu migration V3 takes place in 5 phases.
(If you search the mailing list archive for "migration v3" you will
find a number of 3Ds)
This is a simplified description of the algorithm:
 
      +--+
      |VM|
   +--------+           +---------+
   |Src host|           |dst host |
   +--------+           +---------+

   Phase1: BEGIN
   - Get XML of VM
     and tx it to dst
                         Phase2: PREPARE
                         - create (paused) VM based on XML
   Phase3: PERFORM
   - Copy RAM to dst
   - Pause VM
   ..................................................
                         Phase4: FINISH
                         - Apply port profiles
   ..................................................
                         - Start VM
   Phase5: CONFIRM
   - Kill VM

Libvirt on the dst host orchestrates everything using RPCs.
Dst host can return error/success codes (the two libvirt also
exchange data, .. see cookies)

During phases 1/2/3 the VM is still running on the src host and hence
able to rx/tx traffic.
At the end of Phase 3, when qemu is done copying the RAM from
source host to destination host (*), qemu puts the VM in pause state
on the source host.
(*) See the migration parameter 'migrate-setmaxdowntime'

In phase 4 libvirt applies the port profile to the interfaces
that need it and starts the VM (which had been created and put in
pause state in phase 2).
There is therefore a small period during which both VMs (the one
on the source host and the one on the destination host) are in pause
state (see dotted lines above).
Phase 4 will start the VM on the dst host,
Phase 5 will kill the (paused) VM on the src host.

The amount of time during which both VMs are in pause state depends on
a number of factors, including:
- maxdowntime:  the smaller you configure this value and
                - the longer will take the migration to complete, but
                - the smaller the pause duration will be
- bandwidth available on the interface/s used to carry the migration
  traffic, and amount of RAM assigned to the VM (since you need to
  copy it)
- time taken to complete the port profile associations on the dst host
  (which depends on how loaded the switches are)
- ...


Hope this clarifies a bit.
/Chris

> If this memory state (or traffic) is not copied and moved to the
> destination vNIC how the smooth communication, which is
> guaranteed on VM migration, is achieved?
> 
> Thanks
> Jagath
> 
> >
> > If you refer to port profile info, then there is no copying involved:
> >  the source host disassociates the vnic port profile and
> >  the destination host re-associates the port profile on the new vnic.
> >
> > /Chris
> >
> >> -----Original Message-----
> >> From: libvir-list-bounces redhat com
> > [mailto:libvir-list-bounces redhat com] On Behalf Of Jagath
> >> Weerasinghe
> >> Sent: Friday, April 13, 2012 9:16 AM
> >> To: libvir-list redhat com
> >> Subject: [libvirt] VN-Link vNIC memory state copying on VM Migration
> >>
> >> Hi All,
> >>
> >> I am new to libvirt. And want to know how the VM migration
> >> occurs in VN-Link (IEEE802.1Qbh). As far as I know,
> >> the memory state of vNICs in M81KR VIC has to be copied
> >> and moved to the destination vNIC on VM migration.
> >> Is that correct? If so, could you please tell me how this
> >> has been implemented in libvirt?
> >>
> >> Thanks
> >> Jagath
> >>
> >> --
> >> libvir-list mailing list
> >> libvir-list redhat com
> >> https://www.redhat.com/mailman/listinfo/libvir-list
> 
> --
> libvir-list mailing list
> libvir-list redhat com
> https://www.redhat.com/mailman/listinfo/libvir-list


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