[openstack-dev] [nova] Supporting force live-migrate and force evacuate with nested allocations

Jay Pipes jaypipes at gmail.com
Wed Oct 10 12:46:23 UTC 2018

On 10/10/2018 06:32 AM, Balázs Gibizer wrote:
> Hi,
> Thanks for all the feedback. I feel the following consensus is forming:
> 1) remove the force flag in a new microversion. I've proposed a spec
> about that API change [1]


> 2) in the old microversions change the blind allocation copy to gather
> every resource from a nested source RPs too and try to allocate that
> from the destination root RP. In nested allocation cases putting this
> allocation to placement will fail and nova will fail the migration /
> evacuation. However it will succeed if the server does not need nested
> allocation neither on the source nor on the destination host (a.k.a the
> legacy case). Or if the server has nested allocation on the source host
> but does not need nested allocation on the destination host (for
> example the dest host does not have nested RP tree yet).

I disagree on this. I'd rather just do a simple check for >1 provider in 
the allocations on the source and if True, fail hard.

The reverse (going from a non-nested source to a nested destination) 
will hard fail anyway on the destination because the POST /allocations 
won't work due to capacity exceeded (or failure to have any inventory at 
all for certain resource classes on the destination's root compute node).


> I will start implementing #2) as part of the
> use-nested-allocation-candidate bp soon and will continue with #1)
> later in the cycle.
> Nothing is set in stone yet so feedback is still very appreciated.
> Cheers,
> gibi
> [1] https://review.openstack.org/#/c/609330/
> On Tue, Oct 9, 2018 at 11:40 AM, Balázs Gibizer
> <balazs.gibizer at ericsson.com> wrote:
>> Hi,
>> Setup
>> -----
>> nested allocation: an allocation that contains resources from one or
>> more nested RPs. (if you have better term for this then please
>> suggest).
>> If an instance has nested allocation it means that the compute, it
>> allocates from, has a nested RP tree. BUT if a compute has a nested
>> RP tree it does not automatically means that the instance, allocating
>> from that compute, has a nested allocation (e.g. bandwidth inventory
>> will be on a nested RPs but not every instance will require bandwidth)
>> Afaiu, as soon as we have NUMA modelling in place the most trivial
>> servers will have nested allocations as CPU and MEMORY inverntory
>> will be moved to the nested NUMA RPs. But NUMA is still in the future.
>> Sidenote: there is an edge case reported by bauzas when an instance
>> allocates _only_ from nested RPs. This was discussed on last Friday
>> and it resulted in a new patch[0] but I would like to keep that
>> discussion separate from this if possible.
>> Sidenote: the current problem somewhat related to not just nested PRs
>> but to sharing RPs as well. However I'm not aiming to implement
>> sharing support in Nova right now so I also try to keep the sharing
>> disscussion separated if possible.
>> There was already some discussion on the Monday's scheduler meeting
>> but I could not attend.
>> http://eavesdrop.openstack.org/meetings/nova_scheduler/2018/nova_scheduler.2018-10-08-14.00.log.html#l-20
>> The meat
>> --------
>> Both live-migrate[1] and evacuate[2] has an optional force flag on
>> the nova REST API. The documentation says: "Force <the action> by not
>> verifying the provided destination host by the scheduler."
>> Nova implements this statement by not calling the scheduler if
>> force=True BUT still try to manage allocations in placement.
>> To have allocation on the destination host Nova blindly copies the
>> instance allocation from the source host to the destination host
>> during these operations. Nova can do that as 1) the whole allocation
>> is against a single RP (the compute RP) and 2) Nova knows both the
>> source compute RP and the destination compute RP.
>> However as soon as we bring nested allocations into the picture that
>> blind copy will not be feasible. Possible cases
>> 0) The instance has non-nested allocation on the source and would
>> need non nested allocation on the destination. This works with blindy
>> copy today.
>> 1) The instance has a nested allocation on the source and would need
>> a nested allocation on the destination as well.
>> 2) The instance has a non-nested allocation on the source and would
>> need a nested allocation on the destination.
>> 3) The instance has a nested allocation on the source and would need
>> a non nested allocation on the destination.
>> Nova cannot generate nested allocations easily without reimplementing
>> some of the placement allocation candidate (a_c) code. However I
>> don't like the idea of duplicating some of the a_c code in Nova.
>> Nova cannot detect what kind of allocation (nested or non-nested) an
>> instance would need on the destination without calling placement a_c.
>> So knowing when to call placement is a chicken and egg problem.
>> Possible solutions:
>> A) fail fast
>> ------------
>> 0) Nova can detect that the source allocatioin is non-nested and try
>> the blindy copy and it will succeed.
>> 1) Nova can detect that the source allocaton is nested and fail the
>> operation
>> 2) Nova only sees a non nested source allocation. Even if the dest RP
>> tree is nested it does not mean that the allocation will be nested.
>> We cannot fail fast. Nova can try the blind copy and allocate every
>> resources from the root RP of the destination. If the instance
>> require nested allocation instead the claim will fail in placement.
>> So nova can fail the operation a bit later than in 1).
>> 3) Nova can detect that the source allocation is nested and fail the
>> operation. However and enhanced blind copy that tries to allocation
>> everything from the root RP on the destinaton would have worked.
>> B) Guess when to ignore the force flag and call the scheduler
>> -------------------------------------------------------------
>> 0) keep the blind copy as it works
>> 1) Nova detect that the source allocation is nested. Ignores the
>> force flag and calls the scheduler that will call placement a_c. Move
>> operation can succeed.
>> 2) Nova only sees a non nested source allocation so it will fall back
>> to blind copy and fails at the claim on destination.
>> 3) Nova detect that the source allocation is nested. Ignores the
>> force flag and calls the scheduler that will call placement a_c. Move
>> operation can succeed.
>> This solution would be against the API doc that states nova does not
>> call the scheduler if the operation is forced. However in case of
>> force live-migration Nova already verifies the target host from
>> couple of perspective in [3].
>> This solution is alreay proposed for live-migrate in [4] and for
>> evacuate in [5] so the complexity of the solution can be seen in the
>> reviews.
>> C) Remove the force flag from the API in a new microversion
>> -----------------------------------------------------------
>> 0)-3): all cases would call the scheduler to verify the target host
>> and generate the nested (or non-nested) allocation.
>> We would still need an agreed behavior (from A), B), D)) for the old
>> microversions as the todays code creates inconsistent allocation in
>> #1) and #3) by ignoring the resource from the nested RP.
>> D) Do not manage allocations in placement for forced operation
>> --------------------------------------------------------------
>> Force flag is considered as a last resort tool for the admin to move
>> VMs around. The API doc has a fat warning about the danger of it. So
>> Nova can simply ignore resource allocation task if force=True. Nova
>> would delete the source allocation and does not create any allocation
>> on the destination host.
>> This is a simple but dangerous solution but it is what the force flag
>> is all about, move the server against all the built in safeties. (If
>> the admin needs the safeties she can set force=False and still
>> specify the destination host)
>> I'm open to any suggestions.
>> Cheers,
>> gibi
>> [0] https://review.openstack.org/#/c/608298/
>> [1]
>> https://developer.openstack.org/api-ref/compute/#live-migrate-server-os-migratelive-action
>> [2]
>> https://developer.openstack.org/api-ref/compute/#evacuate-server-evacuate-action
>> [3]
>> https://github.com/openstack/nova/blob/c5a7002bd571379818c0108296041d12bc171728/nova/conductor/tasks/live_migrate.py#L97
>> [4] https://review.openstack.org/#/c/605785
>> [5] https://review.openstack.org/#/c/606111
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