Ideally, a performance problem should be defined within the context of an ongoing performance management process. Performance management refers to the process of establishing performance requirements for applications in the form of a service-level agreement (SLA) and then tracking and analyzing the achieved performance to ensure that those requirements are met. A complete performance management methodology includes collecting and maintaining baseline performance data for applications, systems, and subsystems, for example, storage and network.

In the context of performance management, a performance problem exists when an application fails to meet its predetermined SLA. Depending on the specific SLA, the failure might be in the form of excessively long response times or throughput below some defined threshold.

ESXi and virtual machine (VM) performance tuning are complicated because VMs share the underlying physical resources, in particular, the CPU.

Finally, configuration issues or inadvertent user errors might lead to poor performance. For example, a user might use a symmetric multiprocessing (SMP) VM when a single processor VM would work well. You might also see a situation where a user sets shares but then forgets about resetting them, resulting in poor performance because of the changing characteristics of other VMs in the system.

If you overcommit any of these resources, you might see performance bottlenecks. For example, if too many VMs are CPU-intensive, you might experience slow performance because all the VMs need to share the underlying physical CPU.