System swap is a memory reclamation process that can take advantage of unused memory resources across an entire system.
System swap allows the system to reclaim memory from memory consumers that are not virtual machines. When system swap is enabled you have a tradeoff between the impact of reclaiming the memory from another process and the ability to assign the memory to a virtual machine that can use it. The amount of space required for the system swap is 1GB.
Memory is reclaimed by taking data out of memory and writing it to background storage. Accessing the data from background storage is slower than accessing data from memory, so it is important to carefully select where to store the swapped data.
ESXi determines automatically where the system swap should be stored, this is the Preferred swap file location. This decision can be aided by selecting a certain set of options. The system selects the best possible enabled option. If none of the options are feasible then system swap is not activated.
The available options are:
Datastore - Allow the use of the datastore specified. Please note that a Virtual SAN datastore or a VVol datastore cannot be specified for system swap files.
Host Swap Cache - Allow the use of part of the host swap cache.
Preferred swap file location - Allow the use of the preferred swap file location configured for the host.