Oversubscription in ThinkAgile CP

Oversubscription in ThinkAgile CP refers to various methods by which more resources than are available on the physical host can be assigned to the virtual machines (VMs) that host supports. In general, administrators can oversubscribe processing, memory, and storage resources in virtual machines.

Overprovisioning of processing, memory, and storage can help you maximize resource utilization in your virtual environment. However, with oversubscription, administrators are basically assigning to VMs more resources than are available on the host. In other words, if all of the VMs suddenly requested access to all of their allocated resources, the host would not have enough resources to service these requests. While resource oversubscription increases VM density, it carries some risks. Once an oversubscribed resource is exhausted, stability issues and major performance problems can affect all workloads running on the host server. Therefore, it is important to overprovision carefully and wisely.

Note

Subscription planning should include planning for high-availability events. For example, if a physical compute node is stopped because of maintenance or for failure, the application instances running on it will need to be allocated onto other available resources. If the subscription factor is too high, some or all of the instances will not be able to be run.

• How oversubscription works in ThinkAgile CP
  When compute oversubscription is used, the ThinkAgile CP hypervisor must invoke processor scheduling to distribute processor time to the VMs, or application instances, that need it.
• Oversubscription metrics
  ThinkAgile CP offers oversubscription at two levels.
• Oversubscription considerations
  Oversubscription must be done with care. The conservative way to ensure that your application performs as intended is to not oversubscribe at all. Many administrators follow this approach. However, others consider this approach to be too conservative. Some administrators are comfortable with over committing CPU and storage resources, but not with overcommitting RAM. This is because the real-world consequences of over committing RAM are much worse and more unpredictable than overcommitting CPU.
• Enabling and disabling memory oversubscription
  Memory oversubscription (also known as memory ballooning) is a service daemon that handles dynamic memory allocation across virtual machines (VMs) running on a compute node.