[atomic] 4x I/O speed boost due to vfs --> overlay

[Jeremy Eder <jeder redhat com>]

http://blog.weave.works/2015/07/14/i-quadrupled-the-speed-of-our-container-test-rig-in-a-day/

Workload is I/O heavy, and he somehow ended up with vfs, probably the slowest driver, so 4x is overly optimistic for most folks.

He also made some comments about not using devicemapper with Ubuntu due to the static linking issue...