Scale-out and object-based storage systems are both built for scaling, but metadata characteristics are the difference maker.
What is the best way to define and compare scale-out vs. object-based file systems?
Both types of systems deal with scaling storage, and aren't mutually exclusive. A scale-out file system is typically found in a storage system known as scale-out network-attached storage (scale-out NAS). Here, a single, highly scalable file system is implemented across separate physical modules in a NAS cluster. This helps simplify NAS management without adding new, siloed filers as file shares grow. These are often referred to as single or global namespace file systems.
Object-based storage systems deal with a different issue related to scale -- the ability of the storage system to find data when needed and manage increasingly larger data sets for a broader range of applications. Object-based storage devices (OBSD) add metadata to the file descriptor or create storage data objects that can be defined by their metadata characteristics. Metadata can include content, retention, data protection, security and other types of information the storage system accesses, and can be used to quickly find an object faster as well as automate the management of stored objects. These are important characteristics at larger scale.