[libvirt] [RFC PATCH] Add new migration flag VIR_MIGRATE_DRY_RUN

[Daniel P. Berrangé]

On Mon, Nov 12, 2018 at 11:33:04AM -0700, Jim Fehlig wrote:
> On 11/12/18 4:26 AM, Daniel P. Berrangé wrote:
> > On Fri, Nov 02, 2018 at 04:34:02PM -0600, Jim Fehlig wrote:
> > > A dry run can be used as a best-effort check that a migration command
> > > will succeed. The destination host will be checked to see if it can
> > > accommodate the resources required by the domain. DRY_RUN will fail if
> > > the destination host is not capable of running the domain. Although a
> > > subsequent migration will likely succeed, the success of DRY_RUN does not
> > > ensure a future migration will succeed. Resources on the destination host
> > > could become unavailable between a DRY_RUN and actual migration.
> > 
> > I'm not really convinced this is a particularly useful concept,
> > as it is only going to catch a very small number of the reasons
> > why migration can fail. So you still have to expect the real
> > migration invokation to have a strong chance of failing.
> 
> I agree it is difficult to reliably check that a migration will succeed.
> TBH, I was expecting opposition due to libvirt already providing info for
> applications to do the check themselves. E.g. as nova has done with
> check_can_live_migrate_{source,destination} APIs.
> 
> Do you think libvirt provides enough information for an app to determine if
> a VM can be migrated between two hosts? Or maybe better asked: What info is
> currently missing for an app to reliably check if a VM can be migrated
> between two hosts?

There's probably two classes of problem here

 - Things that would prevent the QEMU process being started.
 
   * XML points to host resources that don't exist (block devices,
     files, nics, host devs, etc, NUMA/CPU pinning)

   * Use of QEMU features that aren't supported by this QEMU version

   * Insufficient free resources. Principally lack of RAM,
     both normal and huge pages.

   These problems are not really anthing todo with live migration
   as they impact normal guest startup to exactly the same degree.

   Libvirt will already report on the first two problems during
   its normal QEMU setup process. During live migration you'll
   see these problems reported quite quickly in the prepare phase
   before any data is sent.

   Insufficient resources is really hard to report on with any
   useful accuracy. We can't even predict reliably how much RAM
   any given QEMU config will need, let alone measure whether
   the host is able to provide that much. If you're lucky QEMU
   may simply fail to start due to insufficient RAM/huge pages.
   This would abort the live migration early on before much data
   is sent.


 - Things that interfere with the live migration operation

    * Firewall blocks libvirtd <-> libvirtd comms

    * Firewall blocks QEMU <-> QEMU comms

    * Storage copy is not requested and disks are not
      on shared storage

    * Network connectivity won't seemlessly switch for
      guest NICs

    * Bugs in QEMU when loading device state causing
      failure

    * Bugs in libvirt not correctly configuring QEMU
      to ensure stable ABI

    * Live migration never converging

   Some of these get seen quite quickly such as firewall
   issues. Bugs in device state are only seen durnig the
   main data transfer. Problems with storage/network
   setup are only seen when the guest crashes & burns
   after migration is complete & are hard to diagnose
   earlier from libvirt's POV. Apps like nova can
   diagnose this kind of thing better as they have a
   higher level view of the storage/network connectivity
   that libvirt can't see.

   Live migration convergance is the real hard one
   that causes alot of pain for people. Personally I
   recommend that people use post-copy by defalt
   to guarantee convergance in finite time, with
   low impact on guest performance. There was an
   interesting presentation at KVM Forum this year
   about doing workload prediction for VMs to identify
   which time/day has a workload that most friendly
   towards convergance.


> > Even launching QEMU isn't good enough - it has to actually process the
> > migration data stream for devices to get a good indication of success,
> > at which point you're basically doing a real migration.
> 
> Bummer. I guess that answers my question above: no. It also implies apps
> cannot reliably check if a migration will succeed and should instead put
> effort into handling errors from an actual migration :-).

Yep, we pretty much have to accept that live migration is going to fail
and work to ensure that when it fails, you don't loose the original
VM. 

Regards,
Daniel
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