Volume sets are an enhancement to VxVM that allow several volumes to be represented by a single logical object. All I/O from and to the underlying volumes is directed via the I/O interfaces of the volume set. The Veritas File System (VxFS) uses volume sets to manage multi-volume file systems and the SmartTier feature. This feature allows VxFS to make best use of the different performance and availability characteristics of the underlying volumes. For example, file system metadata can be stored on volumes with higher redundancy, and user data on volumes with better performance.