DASD background

The mainframe environment can store data on Direct Access Storage Device (DASD) by utilizing an extended count-key-data (ECKD) protocol. DASD is divided into logical units known as volumes. Volumes divide into fixed-size tracks, and the track's geometry determines its size. Linux supports both the 3380 and 3390 track geometries. You can reference a volume by using either a 16-bit device number or a six-character volume serial number (referred to as the volser). More than one operating system can access a volume at the same time. A volume can have a different device number for each operating system that has access to it, but only one volser. Linux allows multiple volumes to have the same volume serial number, while z/OS® allows only one volume per instance of z/OS to have the same volume serial number.