[Bug 1818239] Re: scheduler: build failure high negative weighting

[James Page]

@mriedem - yeah that was my hack but I see you beat me to raising a
review...

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https://bugs.launchpad.net/bugs/1818239

Title:
  scheduler: build failure high negative weighting

Status in OpenStack nova-cloud-controller charm:
  Fix Committed
Status in OpenStack Compute (nova):
  Incomplete
Status in nova package in Ubuntu:
  Triaged

Bug description:
  Whilst debugging a Queens cloud which seems to be landing all new
  instances on 3 out of 9 hypervisors (which resulted in three very
  heavily overloaded servers) I noticed that the weighting of the build
  failure weighter is -1000000.0 * number of failures:

  https://github.com/openstack/nova/blob/master/nova/conf/scheduler.py#L495

  This means that a server which has any sort of build failure instantly
  drops to the bottom of the weighed list of hypervisors for scheduling
  of instances.

  Why might a instance fail to build? Could be a timeout due to load,
  might also be due to a bad image (one that won't actually boot under
  qemu).  This second cause could be triggered by an end user of the
  cloud inadvertently causing all instances to be pushed to a small
  subset of hypervisors (which is what I think happened in our case).

  This feels like quite a dangerous default to have given the potential
  to DOS hypervisors intentionally or otherwise.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: nova-scheduler 2:17.0.7-0ubuntu1
  ProcVersionSignature: Ubuntu 4.15.0-43.46-generic 4.15.18
  Uname: Linux 4.15.0-43-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.5
  Architecture: amd64
  Date: Fri Mar  1 13:57:39 2019
  NovaConf: Error: [Errno 13] Permission denied: '/etc/nova/nova.conf'
  PackageArchitecture: all
  ProcEnviron:
   TERM=screen-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=C.UTF-8
   SHELL=/bin/bash
  SourcePackage: nova
  UpgradeStatus: No upgrade log present (probably fresh install)

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