Large number of collisions when placing instances with anti affinity rules

[Steven Parker]

When deploying heat stacks with anti affinity server groups we are experiencing a large number of collisions when creating instances. Since a heat script is being used for instance creation requests occur in very short time frame increasing the chances of collisions.

This results in rescheduling of these instances increasing stack deployment times.

As the criteria for identical instances often results in identical host selection we would like to be able to configure the filter_scheduler flag "host_subset_size" to a value greater then one to reduce the probability of scheduling collisions and thus reduce rescheduling overall.

This should reduce heat stack deployment times.

This request is related to bug #1817877 which only helps if the placement scores of two or more instances is exactly identical which on our cloud is a rather rare occurrence.

[Steven Parker (sbparke)]

The actual request is to provide the ability in the nova-controller charm to set the filter_scheduler:host_subset_size in nova.conf to an arbitrary number to reduce anti affinity collisions. This parameter would need to be tuneable to discover the optimal setting for a particular cloud.

Thanks,

 Steven

[Tiago Pasqualini da Silva (tiago.pasqualini)]

Patch: https://review.opendev.org/#/c/686843/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/686843
Committed: https://git.openstack.org/cgit/openstack/charm-nova-cloud-controller/commit/?id=bc1d19b680be111b290c94b3e08cd8558bfe94ae
Submitter: Zuul
Branch: master

commit bc1d19b680be111b290c94b3e08cd8558bfe94ae
Author: tpsilva <email address hidden>
Date: Fri Oct 4 18:45:55 2019 -0300

    Add new config option to set host_subset_size

    A new config option called scheduler-host-subset-size was added
    to configure FilterScheduler's host_subset_size property, which
    size of the subset of best hosts selected by the scheduler.

    Change-Id: Ic61917732d50806f74be0b4e7c87a51d738afbe7
    Closes-bug: #1846203