When designing storage for VMware vSphere installations, you have to make a lot of decisions. For instance, there are a number of use cases to have a designated datastore for operating systems. The most frequent use case is to take advantage of primary storage deduplication; this is usually the case with NetApp storage, which guarantees at least a 50% deduplication benefit for virtual machines. Other benefits of this practice include simple organization and basic tiering of the virtual machines.
Why VMware doesn’t recommend datastore layout by OS