When you configure a virtual flash resource to be used by ESXi hosts and virtual machines, several considerations apply.
You can have only one virtual flash resource, also called a VFFS volume, on a single ESXi host. The virtual flash resource is managed only at the host's level.
You cannot use the virtual flash resource to store virtual machines. Virtual flash resource is a caching layer only.
You can use only local flash devices for the virtual flash resource.
You can create the virtual flash resource from mixed flash devices. All device types are treated the same and no distinction is made between SAS, SATA, or PCI express connectivity. When creating the resource from mixed flash devices, make sure to group similar performing devices together to maximize performance.
You cannot use the same flash devices for the virtual flash resource and vSAN. Each requires its own exclusive and dedicated flash device.
The total available capacity of the virtual flash resource can be used by ESXi hosts as host swap cache and by virtual machines as read cache.
You cannot select individual flash devices for swap cache or read cache. All flash devices are combined into a single flash resource entity.