z/OS concepts
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What are DASD volumes and labels?

z/OS concepts

DASD volumes are used for storing data and executable programs (including the operating system itself), and for temporary working storage. DASD labels identify DASD volumes and the data sets they contain.

One DASD volume can be used for many different data sets, and space on it can be reallocated and reused. On a volume, the name of a data set must be unique. A data set can be located by device type, volume serial number, and data set name. This structure is unlike the file tree of a UNIX® system. The basic z/OS® file structure is not hierarchical. z/OS data sets have no equivalent to a path name.

Although DASD volumes differ in physical appearance, capacity, and speed, they are similar in data recording, data checking, data format, and programming. The recording surface of each volume is divided into many concentric tracks. The number of tracks and their capacity vary with the device. Each device has an access mechanism that contains read/write heads to transfer data as the recording surface rotates past them.

The operating system uses groups of labels to identify DASD volumes and the data sets they contain. Customer application programs generally do not use these labels directly. DASD volumes must use standard labels. Standard labels include a volume label, a data set label for each data set, and optional user labels. A volume label, stored at track 0 of cylinder 0, identifies each DASD volume.

The z/OS system programmer or storage administrator uses the ICKDSF utility program to initialize each DASD volume before it is used on the system. ICKDSF generates the volume label and builds the volume table of contents (VTOC), a structure that contains the data set labels. The system programmer can also use ICKDSF to scan a volume to ensure that it is usable and to reformat all the tracks.

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