Disk drives being the performance bottlenecks for any computer systems seem like an eternal truth. There have always been efforts to get the block storage sub-system to deliver more and quicker. Over time, we have seen many caching schemes for performance enhancement; there are block caching solutions that use some sort of cache memory near the disk drive (either on the disk or on the storage controller) or file system level caching solutions that use system memory. Block-level caching with faster block devices has started to look like viable solutions with availability of larger, reliable and economical SSDs.
The Linux open-source community has three generic block-level caching solutions - bcache, dm-cache and flashcache. Patches have been released for all three, but flashcache and bcache efforts appear to have more steam until recently (though that may be changing). Facebook released flashcache and they use it in their production so one may safely say it is the most stable of the three.
Performance studies available for these caching solutions are using SATA or SAS SSDs. Dell PowerEdge Express Flash, being a PCIe SSD, has a much higher performance. It would be interesting to see a performance study for these block caching solutions with PowerEdge Express Flash devices.