A ground-up storage redesign
Upcoming technical changes with positive benefits for you
Duplicated files only use up additional space for the parts you actually change
Apple Filing System (APFS) is the new storage format which all four of Apple’s operating systems will eventually use. While it isn’t fully used in Sierra, Apple has provided an early version to developers to test in the new OS; APFS supports almost all the same features as HFS+, the Mac’s file system since the late ’90s, but Apple explicitly warns that disk utilities will need to be updated for APFS. So, if you depend on anything of that nature, start saving for an upgraded version.
The reason for APFS’s development stems from technical attributes of HFS+ that are a clue to its age; HFS+ dates from a time when magnetic media was the dominant type, whereas APFS will be optimized for the flash storage that’s used in most Apple devices today. The details Apple says that security and privacy are fundamental in APFS’s design. A disk can be formatted either without encryption, or using a single key for the whole of it, or with a different key for each file. APFS’s cloning feature means duplicates take up no extra space until you change one version, at which point only modified blocks are stored, rather than a copy of the whole file, which could save you a lot of space. Then there are snapshots, which are read-only copies of files that enable rolling back to an older version.
APFS itself doesn’t implement RAID (a method of pooling physical disks for better performance or redundancy), but Apple says you can combine it with macOS’s software RAID implementation. Also, where Disk Utility lost graphical RAID management in El Capitan, Sierra brings it back. But Apple has deprecated the AFP protocol used for network file sharing, so using APFS on your NAS will need the drive to support SMB or NFS.