Bcache allows for one or more solid-state drives (SSDs) to act as a cache for slower rotational hard disk drives. Write-back and write-through caching models are supported by Bcache while also working at the block level so it supports any file-system, unlike L2Arc with the ZFS file-system.
It's been nearly one year since last having anything to write about with Bcache, but following some progress over the winter holiday, the support might finally soon its way into the mainline Linux kernel.
Kent Overstreet, the principal Bcache developer, wrote on the kernel mailing list, "Over the Christmas break I finally got the tree into a self contained state that ought to be suitable for merging; this tree is fairly close to the previous stable tree that people have been running on production servers for awhile (and that I've been running on this workstation), So, I think this is ready for mainline and I'd like to get it in..."
For more information on Linux Bcache, see its Wiki page.
In related news, you might also want to read about EnhanceIO for SSD caching on Linux. EnhanceIO is commercial code recently opened up and based upon Facebook's Flashcache for providing similar functionality to Bcache. EnhanceIO is also open-source (GPL-licensed) and has mainline Linux kernel ambitions.