Comparing DASD and SDDD
Data Access Storage Device (DASD) and Storage Direct-Access Device (SDDD) are both used to provide file storage in computer systems. Both technologies involve spinning disks, but each achieves a different result:
DASD is used to store data files, such as text and images. The disk is designed to appear as one continuous stream of data to the operating system. DASD is the most commonly used type of file storage and is most often found in traditional hard disk drives.
SDDD, on the other hand, is used to store data records, such as database and system files. In contrast to DASD, the disk is designed to appear as multiple separate blocks of data to the operating system, allowing the user to directly access individual records. SDDD is most often found in modern RAID systems.
One major difference between these two technologies is the way the operating system interacts with them. With DASD, the operating system only needs to know the general layout of the disk and can access the data through the filesystem. With SDDD, the operating system needs to know specific details about the physical layout of the disk, such as the number of blocks per track and the number of tracks per cylinder. This extra information allows the operating system to access individual records directly.
Another major difference concerns the way the disks are organized. DASD traditionally organizes data in a linear fashion, with the data stored in a continuous stream from one side of the disk to the other. SDDD, on the other hand, organizes data in what is known as a “Zigzag” format, with the records stored in groups of adjacent tracks. This allows individual records to be accessed more quickly, but it also requires the operating system to know the exact location of each record.
Overall, the main difference between DASD and SDDD is the way the operating system interacts with them. Both technologies involve spinning disks, but the operating system accesses them in different ways. DASD is used to store data files, while SDDD is used to store data records. Both technologies have their advantages and disadvantages, and the best option depends on the needs of the application.