title: Why use AUFS as the default Docker storage backend instead of devicemapper?
body: <p>Project Atomic's description of Docker <a href="http://www.projectatomic.io/docs/filesystems/">storage backends</a> describes technical differences between AUFS and other storage backend choices, such as devicemapper.  <a href="http://aufs.sourceforge.net">AUFS</a> is not in the upstream Linux kernel.  Why is AUFS chosen as the default storage backend (for example in Ubuntu's Docker)? Are there some technical properties of AUFS that makes it a better choice than a storage backend such as devicemapper that seems to be supported by an unpatched Linux kernel?</p>