Yes, it is possible to extend support to BTRFS for the Hummock tiered cache. The current tiered cache implementation uses syscall fallocate with FALLOC_FL_PUNCH_HOLE, which is only supported by limited file systems. BTRFS actually supports the flag, but it requires some additional configuration. The file cache bench tool needs to manually specify the device to monitor for BTRFS, as it can span across multiple block devices.