Why do undersized VMs lead to latency and other problems?

Having undersized VMs in a virtualization system is a classic example of improper resource allocation that leads to concrete performance issues.

In systems with undersized VMs, users will experience latency, diminished service, and even problems with screen hang or freeze. This is, simply, because the virtual machine has not been given a sufficient amount of CPU and memory resources to do its job properly. Often, this problem results from planning that has not capably addressed the real operational needs of the VM for some reason. There can be a significant amount of guesswork in setting up a virtualization system and predicting how many resources a VM might use. There is also the issue of dynamic demand, where a particular VM might encounter more user demand or have to scale up quickly.

Undersized VMs are by no means the only reason for system latency. That said, it can be difficult to diagnose the issue. Latency may come from bottlenecks elsewhere in the system, or a lack of sufficient DRAM, or even clock synchronization or driver issues.

With this in mind, vendors have created vibrant network monitoring and control systems that automate the process of allocating resources to VMs. Many of these systems have color-coded dashboards showing whether a virtual machine or component is undersized or lacks any resource. These systems can also provide similar diagnostic tools for clusters of VMs on a host.

Just as these automation systems can help deal with the issue of undersized VMs, they can also address the opposite issue of oversized VMs. Oversized VMs will not generally manifest issues with performance – instead of having too few resources on hand, they have too many. However, as some experts point out, an oversized VM can lead to latency on other starved VMs elsewhere in the system.

In order to eliminate all of these kinds of problems, companies have adopted the practice of “right-sizing” VMs and everything else in a virtualization environment. This can be done manually, or with the above-mentioned automation systems. Doing this work manually will take important human resources from a business model, which is why so many companies choose to utilize a vendor tool for right-sizing.