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What is oversubscription?

Question:

What is oversubscription and how does it affect my environment?

Answer:

Subscription is defined as the total number of virtual processors or amount of memory allocated to all powered-on virtual machines in a compute pool. Oversubscription is when the subscribed resources exceed those purchased.

Note: Oversubscription is not applicable to instance-based compute pools.

Processor oversubscription

Processor oversubscription is defined as the total number of virtual processors assigned to all powered on virtual machines, multiplied by the host process core speed. If this number is greater than the GHz purchased, the compute pool is said to be oversubscribed.

Processor oversubscription is not usually a major factor in noted performance issues, but can cause some issues.

Best practice is to have 1 processor per GHz of processing power allocated or purchased. See VMware Performance Tuning Best Practices – Processor for more information.

Even though the virtual machines' processor usage is showing as minimal and not using the full amount of the host processing power, the resource scheduler sees the request and has to assign a host processor core to each virtual processor. Therefore, when a 10 GHz compute pool has a total of 15 vCPUs assigned, some performance degradation may occur depending on the amount and type of tasks occurring as the resource scheduler sees the need for 15 cores and has to move the processing power between the virtual machines.

Oversubscription can be offset by use of Bursting, but if the oversubscription is more than 25%, enabling Burst Mode will have little to no effect on performance.

Memory oversubscription

Memory oversubscription is one of the most common causes of noted performance issues and non-responsive virtual machines.

Best practice is to only allocate the amount of memory required by the application with the highest memory requirements. See VMware Performance Tuning Best Practices – Memory for more information.

When memory oversubscription occurs, the virtual machine kernel is forced to reclaim the memory as needed through:

  • Ballooning – vmmemctl driver collaborates with the server to reclaim pages that are considered least valuable by the guest operating system, forcing the guest operating system to invoke its own native memory algorithms.
  • Swapping – Use of disk space swap file instead of RAM.

Therefore, when a 100 GB Memory compute pool has 250 GB memory allocated and powered on, the balloon driver activates and causes disk swapping.

Oversubscription can be offset by use of bursting, but if the compute pool is oversubscribed by more than 25%, enabling burst mode will have little to no effect on performance.

Storage oversubscription

Storage is a static resource consumed when allocated and therefore cannot be oversubscribed.