volume 就是一个存储设备，Volumn Space 就是这个存储设备的空间，而 File Space 就是File的Size。
A volume is a storage device, such as a fixed disk, floppy disk, or CD-ROM, that is formatted to store directories and files. A large volume can be divided into more than one logical volume, also called a partition. Each logical volume is formatted for use by a particular media-based file system, such as NTFS, FAT, or CDFS.
A storage volume, or storage device object, is a device object − usually a physical device object (PDO) − that represents a logical volume to the system. The storage device object resides in the storage device stack, but it is not necessarily the topmost device object in the stack.
When a file system is mounted on a storage volume, it creates a file system volume device object (VDO) to represent the volume to the file system. The file system VDO is mounted on the storage device object by means of a shared object called a volume parameter block (VPB).
In computers, a volume is an identifiable unit of data storage that is sometimes (but not always) physically removable from the computer or storage system. In tape storage systems, a volume may be a tape cartridge (or, in older systems, a tape reel). In mainframe storage systems, a volume may be a removable hard disk. Each volume has a system-unique name or number that allows it to be specified by a user.
In some systems, the physical unit may be divided into several separately identifiable volumes.
引用文档：
https://msdn.microsoft.com/en-us/library/windows/hardware/ff556757(v=vs.85).aspx