Create a RAID 0 filesystem where an SSD acts as a cache and an HDD acts as a long-term storage
sudo bcachefs format --label=ssd.ssd1 {{path/to/ssd/partition}} --label=hdd.hdd1 {{path/to/hdd/partition}} --replicas=1 --foreground_target=ssd --promote_target=ssd --background_target=hdd