Undersized & Oversized VM’s – Demystified

Undersized VMs

Definition: – You have allocated certain amount of virtual hardware resources to a virtual machine and the resources are not sufficient for the VM.

Impact: This would manifest in performance issues such a high latency/slow responses/sluggish behaviour/freeze/hang.

Oversized VMs

Definition: – You have allocated certain amount of virtual hardware resources to a virtual machine and the resources are highly underused or unused.

Impact: Mismanaged capacity-Although this does not visually manifest there would be other VM’s that may be suffering because of the VM that is oversized.

 

Calculation Mechanism for Oversized VMs

Threshold for CPU demand & Memory demand is calculated against time.

If the utilization is below threshold over a period of time, the VM is deemed as Oversized

Calculation Mechanism for Undersized VMs

Threshold for CPU demand & Memory demand is calculated against time.

If the Threshold for resource exceeds the threshold of time period, the VM is deemed as Oversized

Configuring it right

1-Log in to vSphere UI – <ipaddress of UI VM/vcops-vpshere>

2-Click on Configuration

Config

Note: Choose or Create a policy and map it to a specific group to which the policy is appropriate

3-Click on 4b-Oversized and undersized VMs


4- Profile Virtual Machine loads and end user access patterns

5-Check if the application is staggered workload or consistent workload

6- Configure the threshold to fit the expected “business hours” utilization for that Virtual Machine/Group

For instance a backup VM will be active during off business hours and for a finite window, this is an example of staggered load. This would commonly be reported as an oversized VM.

Caution

  • One size does not fit all, Ensure you categorize types of workload and carve out policies for each category
  • Do not enforce changes purely based undersized and oversized reports.
  • VCOPS is unaware of Guest OS or application requirements and computes the values for VM as a whole.
  • Application might crash is the minimum requirement or configuration needs are not met.

Take it with a pinch of salt !!!

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