Currently you have raid1 as only raid-profile option for the cache disks.
It would be nice, if you can choose between all possible btrfs raid-profiles.
Examples:
2 x 250GB SSDs:
raid1-profile:
full Capacity: 250GB, but safety.
raid0-profile:
full Capacity: 500GB, no safety.
OR
1 x 250GB SSD
1 x 500GB SSD
raid1-profile:
full Capacity: 250GB, but safety and 250GB wasted.
raid0-profile:
full Capacity: 250GB, no safety and 250GB wasted.
single-profile:
full Capacity: 750GB, no safety.
I have a 250GB and a 500GB SSD, if I want to use the full capacity i have to use the single profile for the data blocks, rather than raid0 but this is not possible, currently.