Wikipedia
Flashcache
Flashcache is a disk cache component for the Linux kernel, initially developed by Facebook since April 2010, and released as open source in 2011. Since January 2013, there is a fork of Flashcache, named EnhanceIO and developed by sTec, Inc.
Flashcache works by using flash memory, a USB flash drive, SD card, CompactFlash or any kind of portable flash mass storage system as a write-back persistent cache. An internal SSD can also be used for increasing performance.