Do you need a reservation or a resource pool? Don’t know? Well hopefully this will help you decide which one will best fit your need.
Lets start with a reservation. A reservation of cpu or memory is basically what it sounds like. It reserves the minimum amount of resources you set exclusively to that virtual machine. Once reserved it can’t be used by any other virtual machine. While this seems like a great thing to do in order to guarantee resourses to an application it also has it’s down side. First off you remove the concept of shared resources when you use reservations and you affect the slot size of a cluster with HA configured. In both of these cases you limit the number of virtual machines that can be hosted in your environment so use reservations wisely.
Then there is the resource pool. The resource pool is like a folder that has quotas configured on it. You create a resource pool, in most cases, to set boundaries for multiple vm guests that are placed within that resource pool and these boundaries can be hard or soft. The boundaries consist of how much cpu and memory the vm guests are allotted to use as a whole by the one or many virtual machines in the resource pool. In this case the resources are not reserved so if the pool does not use the resources then there free for all to consume, even those outside of the pool.
This was short an to the point but hopefully helpful in your decision on whether to use resource pools or reservations. Both can be configured as well so you don’t have to settle for just one or the other in your environment. More detail can be found in the Resource Management Guide from VMware.