Properly calculate a cluster's free memory

[cunnie]

cluster free memory in the vSphere CPI uses "effective_memory" [0] defined as follows [1]:

Effective memory resources (in MB) available to run virtual
machines. This is the aggregated effective resource level from all
running hosts. Hosts that are in maintenance mode or are unresponsive
are not counted. Resources used by the VMware Service Console are not
included in the aggregate. This value represents the amount of resources
available for the root resource pool for running virtual machines.

This sounds like it's "available free memory", but it's not. It's total effective memory that a cluster could give to VMs. This is the wrong metric to use for "free memory" as it does not vary based on the memory consumption for the cluster.

This commit changes the calculation to subtract from the effective memory the memory demand, "Sum of memory demand of all the powered-on VMs in the cluster" [2].

The new metric appears to more closely reflect vCenter's reporting of memory. For example, on https://vcenter-80.nono.io:

   490 GiB   effective memory, old, wrong calculation
  - 86 GiB   demand memory
   -------
   404 GiB   new, better calculation

   420 GiB   vCenter's reporting of "Available Reservation"

Note that although this new calculation of memory is more "truthful", measuring memory is nuanced: vSphere has 3 different metrics for memory use (memDemandMB, memEntitledMB, memReservationMB). Furthermore, on vCenter's cluster's summary page, the "free" memory (e.g. 184 GiB) doesn't dovetail with any of the available metrics, nor any combination thereof.

Drive-by:

• I changed VimSdk::Vim::ComputeResource::Summary → VimSdk::Vim::ClusterComputeResource::Summary in our tests because that was the object reflected in the vSphere MOB browser (we were instance-doubling the wrong object).

[0]

bosh-vsphere-cpi-release/src/vsphere_cpi/lib/cloud/vsphere/resources/cluster.rb

Lines 209 to 216 in 3fc6d72

	def fetch_cluster_utilization()
	logger.debug("Fetching Memory utilization for Cluster #{self.mob.name}")
	properties = @client.cloud_searcher.get_properties(mob, Vim::ClusterComputeResource, 'summary')
	raise "Failed to get utilization for cluster'#{self.mob.name}'" if properties.nil?
	compute_resource_summary = properties["summary"]
	return compute_resource_summary.effective_memory
	end

[1] https://vdc-download.vmware.com/vmwb-repository/dcr-public/90ec343b-df7c-493e-9979-36ea55765102/8753fd1e-fcab-4bd4-9cde-a364851f31a6/vim.cluster.UsageSummary.html

[2] https://vdc-download.vmware.com/vmwb-repository/dcr-public/3d076a12-29a2-4d17-9269-cb8150b5a37f/8b5969e2-1a66-4425-af17-feff6d6f705d/SDK/sms-sdk/docs/ReferenceGuide/vim.ComputeResource.Summary.html#effectiveMemory

Suggested release notes:

[Bug Fix] The amount of available RAM available in a vSphere cluster is now calculated more conservatively. This should have no affect other than in memory-starved clusters, in which case errors will be raised earlier in the deployment phase.

Description

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• My code follows the standard ruby style guide
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• I have added tests that prove my fix is effective or that my feature works
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[aramprice]

@cunnie - is this still in flight, or should this be picked up by someone else?

[cunnie]

@aramprice

is this still in flight, or should this be picked up by someone else?

It should be picked up by someone else, but I don't know how important it is given that users have gotten along just fine (so far as I know).