VMware also supports Raw Device Mapping (RDMs) and NFS as alternatives to VMDKs.
Raw Device Maps or RDMs allow volumes from ScaleIO to be presented directly to the guest operating system and to largely bypass the hypervisor.
With RDMs, a mapping file is still created in VMFS, but all reads and writes go directly to the volume on ScaleIO.
RDMs come in two versions: Virtual Mode RDMs (vRDMs) and Physical Mode RDMs (pRDMs) which are also sometimes referred to as pass-thru RDMs.
pRDMs may be used to create hybrid clusters with some nodes virtual and others physical.
The largest pRDM supported in vSphere 6 is 64 TB.
Contrary to some published information, both VMDKs and pRDMs allow the use of VM vMotion, VM HA and VM DRS, although certain restrictions apply.
During testing, Dell EMC has found that VMFS incurs a performance penalty of between 5 percent to 13 percent compared to using RDMs. Note that some non-Dell EMC published reports have found little or no performance difference between VMFS and RDMs.