This code, which was formerly commercial and then opened up STEC Inc, is based upon Facebook's Flashcache. EnhanceIO is meant to treat solid-state drives as cache devices to traditional rotating hard drives. Many improvements to EnhanceIO were made over Flashcache and it's now getting ready to enter the mainline Linux kernel.
Darrick Wong, an Oracle developer that commonly is working on file-systems for the Linux kernel, is one of the developers working to merge this code. On Friday, Darrick published a patch that he's seeking comments on that is working towards migrating STEC's EnhanceIO driver out of their GitHub repository and readying it for entering the Linux kernel staging tree.
The EnhanceIO driver hasn't been cleaned up yet and made to the standards needed for entering the kernel, but he's seeking the patch be looked over under an "RFC" state. For those interested, the EnhanceIO SSD caching driver patch can be found for now on the kernel mailing list. It's possible the patch could be readied for the staging area in the Linux 3.9 kernel, but it might not be until Linux 3.10 (or even later).