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[Dan Smith <dms@xxxxxxxxxxxxx>]

Public bug reported:

Nova's resource tracker is expected to publish negative values to the
scheduler when resources are overcommitted. Nova's scheduler expects
this:

https://github.com/openstack/nova/blob/a43dbba2b8feea063ed2d0c79780b4c3507cf89b/nova/scheduler/host_manager.py#L215

In change https://review.openstack.org/#/c/306670, these values were
filtered to never drop below zero, which is incorrect. That change was
making a complex alteration for ironic and cells, specifically to avoid
resources from ironic nodes showing up as negative when they were
unavailable. That was a cosmetic fix (which I believe has been corrected
for ironic only in this patch:

https://review.openstack.org/#/c/230487/

Regardless, since the scheduler does the same calculation to determine
available resources on the node, if the node reports 0 when the
scheduler calculates -100 for a given resource, the scheduler will
assume the node till has room (due to oversubscription) and will send
builds there destined to fail.

** Affects: nova
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1698383

Title:
  Resource tracker regressed reporting negative memory

Status in OpenStack Compute (nova):
  New

Bug description:
  Nova's resource tracker is expected to publish negative values to the
  scheduler when resources are overcommitted. Nova's scheduler expects
  this:

  https://github.com/openstack/nova/blob/a43dbba2b8feea063ed2d0c79780b4c3507cf89b/nova/scheduler/host_manager.py#L215

  In change https://review.openstack.org/#/c/306670, these values were
  filtered to never drop below zero, which is incorrect. That change was
  making a complex alteration for ironic and cells, specifically to
  avoid resources from ironic nodes showing up as negative when they
  were unavailable. That was a cosmetic fix (which I believe has been
  corrected for ironic only in this patch:

  https://review.openstack.org/#/c/230487/

  Regardless, since the scheduler does the same calculation to determine
  available resources on the node, if the node reports 0 when the
  scheduler calculates -100 for a given resource, the scheduler will
  assume the node till has room (due to oversubscription) and will send
  builds there destined to fail.

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