On Tue, Oct 9, 2018 at 4:56 PM, Sylvain Bauza <sylvain.ba...@gmail.com> wrote: > > > Le mar. 9 oct. 2018 à 16:39, Eric Fried <openst...@fried.cc> a > écrit : >> IIUC, the primary thing the force flag was intended to do - allow an >> instance to land on the requested destination even if that means >> oversubscription of the host's resources - doesn't happen anymore >> since >> we started making the destination claim in placement. >> >> IOW, since pike, you don't actually see a difference in behavior by >> using the force flag or not. (If you do, it's more likely a bug than >> what you were expecting.) >> >> So there's no reason to keep it around. We can remove it in a new >> microversion (or not); but even in the current microversion we need >> not >> continue making convoluted attempts to observe it. >> >> What that means is that we should simplify everything down to ignore >> the >> force flag and always call GET /a_c. Problem solved - for nested >> and/or >> sharing, NUMA or not, root resources or no, on the source and/or >> destination. >> > > > While I tend to agree with Eric here (and I commented on the review > accordingly by saying we should signal the new behaviour by a > microversion), I still think we need to properly advertise this, > adding openstack-operators@ accordingly.
Question for you as well: if we remove (or change) the force flag in a new microversion then how should the old microversions behave when nested allocations would be required? Cheers, gibi > Disclaimer : since we have gaps on OSC, the current OSC behaviour > when you "openstack server live-migrate <target>" is to *force* the > destination by not calling the scheduler. Yeah, it sucks. > > Operators, what are the exact cases (for those running clouds newer > than Mitaka, ie. Newton and above) when you make use of the --force > option for live migration with a microversion newer or equal 2.29 ? > In general, even in the case of an emergency, you still want to make > sure you don't throw your compute under the bus by massively > migrating instances that would create an undetected snowball effect > by having this compute refusing new instances. Or are you disabling > the target compute service first and throw your pet instances up > there ? > > -Sylvain > > > >> -efried >> >> On 10/09/2018 04:40 AM, Balázs Gibizer 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 >> > >> > >> > >> __________________________________________________________________________ >> > OpenStack Development Mailing List (not for usage questions) >> > Unsubscribe: >> openstack-dev-requ...@lists.openstack.org?subject:unsubscribe >> > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev >> > >> >> __________________________________________________________________________ >> OpenStack Development Mailing List (not for usage questions) >> Unsubscribe: >> openstack-dev-requ...@lists.openstack.org?subject:unsubscribe >> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev __________________________________________________________________________ OpenStack Development Mailing List (not for usage questions) Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
