Flash Read Cache (vFlash) is a feature, introduced in vSphere 5.5 and available in the Enterprise Plus edition, that can improve virtual machine storage performance by using host local flash devices as a cache. The performance boost depends on your workload type and working set size. Only read-intensive workloads, with working sets that fit into the cache size, can really benefit from the Flash Read Cache feature. vSphere Flash Read Cache offers legacy support for the swap-to-SSD feature introduced in vSphere 5.0; that was a previous way to use a local SSD to host VM-related swap files.

You can reserve a Flash Read Cache for any individual virtual disk that is created only when a virtual machine is powered on; it is discarded when a virtual machine is suspended or powered off. When you migrate a virtual machine, you can migrate the cache (default option); otherwise, if you do not migrate the cache, the cache is rewarmed on the destination host. Flash Read Cache does not support RDMs in physical compatibility. Virtual compatibility RDMs are supported with Flash Read Cache as also vSphere HA and DRS; but not FT.

VM cache configuration could be managed also from the vSphere Client (HTML5), but without the advanced settings, available only in the vSphere Web Client:

Host configuration can only be managed with vSphere Web Client. You need to add a free local SSD disk to the capacity pool.

Flash Read Cache supports only write-through or reads caching; write-back, or write caching are not supported at all. But there are other products that can support it.