Datastores that are created on solid state drives (SSD) can be used to allocate space for host cache. The host can reserves a certain amount of space for swapping to host cache. It is configurable as shown below
The host cache is made up of files equally divide into 1 GB size.
ESXi uses this file as write back cache for virtual machine swap files. The cache is shared by all virtual machines running on the host.
If you analogy of storage which has both read and write cache, it applies to host cache. Here VMware has only enhanced this cache by using SSD disk for caching.
Therefore using swap to host cache is not the same as placing regular swap files on SSD-backed datastores. Even if you enable swap to host cache, the host still needs to create regular swap files. However, when you use swap to host cache, the speed of the storage where the host places regular swap files is less important.
Note Only SSD-backed datastores appear in the list of datastores on the Host Cache Configuration page.
Design considerations for swapping to host cache (SSD) is available at www.yellow-bricks.com
Bottom line is swapping to cache will happen only when there is severe memory pressure.