You can aggregate local flash devices on an ESXi host into a single virtualized caching layer called virtual flash resource.
When you set up the virtual flash resource, you create a new file system, Virtual Flash File System (VFFS). VFFS is a derivative of VMFS, which is optimized for flash devices and is used to group the physical flash devices into a single caching resource pool. As a non-persistent resource, it cannot be used to store virtual machines.
The following vSphere functionalities require the virtual flash resource:
Virtual machine read cache. See About VMware vSphere Flash Read Cache.
Host swap cache. See Configure Host Swap Cache with Virtual Flash Resource.
I/O caching filters, if required by your vendors. See Filtering Virtual Machine I/O.
Before setting up the virtual flash resource, make sure that you use devices approved by the VMware Compatibility Guide.