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 ...