A: The VMFS file system gets a major version update in VMware vSphere 5, increasing its version counter from VMFS-3 to VMFS-5. Although it's entirely possible to upgrade existing VMFS-3 partitions non-disruptively, doing that upgrade might actually create a downstream problem to the operations of Storage DRS.
Recall that VMFS-3 partitions in vSphere 4.x could be created with any of four different block sizes. Those block sizes ranged from 1MB to 8MB. However, in vSphere 5, newly created volumes are always created with 1MB block sizes.
When a VMFS-3 partition is upgraded in place, its block size doesn't change. This situation can mean that different volumes with different block sizes can later be gathered into a Storage DRS cluster. When this occurs, Storage DRS is forced to use a less-efficient copying mechanism when migrating virtual machine (VM) disk files between volumes of different block sizes.
As a result, VMware recommends you create new VMFS-5 volumes for those that participate in a Storage DRS cluster rather than upgrading existing volumes from VMFS-3.