If you have a machine with slow hard disks and fast SSDs, and you want to use the SSDs to act as fast persistent caches to speed up access to the hard disk, then until recently you had three choices: bcache and dm-cache are both upstream, or Flashcache/EnhanceIO. Flashcache is not upstream. dm-cache required you to first sit down with a calculator to compute block offsets. bcache was the sanest of