At the infrastructure level, you look at the big picture. Hence, it is important that you know your architecture well. One way to easily remember what you have is to keep it simple. Yes, you can have different host specifications—CPU speed, amount of RAM, and so on in a cluster. But, that would be hard to remember if you have a large farm with many clusters.

You also need to know what you actually have at the physical layer. If you don't know how many CPUs or how much RAM the ESXi host has, then it's impossible to figure out how much capacity is left. I will use storage as an example to illustrate why this is important. Do you know how many IOPS your storage has?

The majority of shared storage is shared with both ESXi and non-ESXi servers mounted. Even if the entire storage is dedicated to ESXi, there is still the physical backup server mounting it, and it might be doing array-based replication or a snapshot.

Some storage support dynamic tiering...