Using Direct Access Storage Devices (DASD)

Direct access storage devices accept records of format-F, format-V, or format-U. To read or write the records with keys, you must specify the key length (KEYLEN). In addition, the operating system has a standard track format for all direct access volumes. See Track Format for a complete description of track format. Each track contains data information and certain control information, such as the following information:

The address of the track
The address of each record
The length of each record
Gaps between areas

Except for a PDSE or compressed format data set, the size of a block cannot exceed what the system can write on a track. For PDSEs and compressed format data sets, the access method simulates blocks, and you can select a value for BLKSIZE without regard to the track length. A compressed format data set is a type of extended format data set that is stored in a data format that can contain records that the access method compressed.