vSphere 5.5 introduced the vSphere Flash Read Cache that enables the pooling of multiple Flash-based devices into a single consumable vSphere construct called vSphere Flash Resource.
vSphere hosts can use the vSphere Flash Resource as vSphere Flash Swap Cache, which replaces the Swap to SSD feature previously introduced with vSphere 5.0. It provides a write-through cache mode that enhances virtual machines performance without the modification of applications and OSs.
At its core Flash Cache enables the offload of READ I/O from the shared storage to local SSDs, reducing the overall I/O requirements on your shared storage.

Documented maxima with vSphere 6.5:
- Virtual flash resource per host: 1
- Maximum cache for each virtual disk: 400GB
- Cumulative cache configured per host (for all virtual disks): 2TB
- Virtual disk size: 16TB
- Virtual host swap cache size: 4TB
- Flash devices (disks) per virtual flash resource: 8