MacOS Sierra Features: Apple File System
Besides of having new fancy features like Siri, MacOS Sierra also included very important under hood improvements such as the new Apple File System.
Starting from 2017, HPS+ file system will be replaced by APFS not only on MacOS but also on iOS, tvOS, and watchOS. However, APFS system is already included in the MacOS Sierra Beta.
APFS File system has all the features of HPS+ but it is greatly optimized for Flash/SSD storage, encryption, copy-on-write metadata and fast directory sizing.