Direct Access Storage Device Architecture

Disks reside in direct access storage subsystems. The real disks might have an architecture that differs from what the subsystem presents to the operating system. The operating system sees direct access storage devices (DASDs). This document and other z/OS documentation describe DASDs. Hardware documentation describes internal characteristics of direct access storage subsystems.

As seen by software, each disk or tape is called a volume. Each volume can contain one or more complete data sets and parts of data sets. Each complete or partial data set on a DASD volume has a data set label. Each complete or partial data set on a tape volume has a data set label only if the volume has IBM standard labels or ISO or ANSI standard labels. For information about data sets and labels on magnetic tapes, see Magnetic Tape Volumes.

Only standard label formats are used on direct access volumes. Volume, data set, and optional user labels are used (see Figure 1). In the case of direct access volumes, the data set label is the data set control block (DSCB).

The system programmer or storage administrator uses ICKDSF to format tracks, write a volume label, and create a volume table of contents (VTOC). The VTOC contains all the DSCBs. RACF DASDVOL authority is required to create a VTOC. DASDVOL authority is not required to allocate space on volumes. The system controls space on SMS volumes by other means such as the ACS routines, storage group definitions and ISMF commands.

Related reading: For more information about tracks and records, see Direct Access Storage Device (DASD) Volumes.

Figure 1. Direct Access Labeling