another FUSE union filesystem

 mergerfs is a union filesystem geared towards simplifying storage and
 management of files across numerous commodity storage devices. It is
 similar to mhddfs, unionfs, and aufs.
 .
 Some salient features include
  * Runs in userspace (FUSE)
  * Configurable behaviors
  * Support for extended attributes (xattrs)
  * Support for file attributes (chattr)
  * Runtime configurable (via xattrs)
  * Safe to run as root
  * Opportunistic credential caching
  * Works with heterogeneous filesystem types
  * Handling of writes to full drives
  * Handles pool of readonly and read/write drives