A cache I/O filter can use a local flash device to cache virtual machine data.
If your caching I/O filter uses local flash devices, before activating the filter, you need to configure a virtual flash resource, also known as VFFS volume, on your ESXi host. While processing the virtual machine read I/Os, the filter creates a virtual machine cache and places it on the VFFS volume.
To set up a virtual flash resource, you use flash devices that are connected to your host. To increase the capacity of your virtual flash resource, you can add more flash drives. An individual flash drive must be exclusively allocated to a virtual flash resource and cannot be shared with any other vSphere service, such as Virtual SAN or VMFS.
Flash Read Cache and caching I/O filters are mutually exclusive because both functionalities use the virtual flash resource on the host. You cannot enable Flash Read Cache on a virtual disk with the cache I/O filters. Similarly, if a virtual machine has Flash Read Cache configured, it cannot use the cache I/O filters.