IBM Direct access storage devices (DASD)

The first disk drives for the 360 were IBM 2302s:60-65 and IBM 2311s. The 156 kB/second 2302 was based on the earlier 1302 and was available as a model 3 with two 112.79 MB modules or as a model 4 with four such modules.

The 2311, with a removable 1316 disk pack, was based on the IBM 1311 and had a theoretical capacity of 7.2 MB, although actual capacity varied with record design.31

In 1966, the first 2314s shipped. This device had up to eight usable disk drives with an integral control unit; there were nine drives, but one had to be reserved as a spare. Each drive used a removable 2316 disk pack with a capacity of nearly 28 MB. The disk packs for the 2311 and 2314 were physically large by today's standards — e.g., the 1316 disk pack was about 14 in (36 cm) in diameter and had six platters stacked on a central spindle. The top and bottom outside platters did not store data. Data were recorded on the inner sides of the top and bottom platters and both sides of the inner platters, providing 10 recording surfaces. The 10 read/write heads moved together across the surfaces of the platters which were formatted with 203 concentric tracks. To reduce the amount of head movement (seeking), data was written in a virtual cylinder from inside top platter down to inside bottom platter. These disks were not usually formatted with fixed-sized sectors as are today's hard drives (though this was done with CP/CMS). Rather, most S/360 I/O software could customize the length of the data record (variable-length records), as was the case with magnetic tapes.

IBM 2314 Disk Drives and IBM 2540 Card Reader/Punch at the University of Michigan.

Some of the most powerful early S/360s used high-speed head-per-track drum storage devices. The 3,500 RPM 2301,which replaced the 7320, was part of the original S/360 announcement, with a capacity of 4Mb. The 303.8 kB/second IBM 2303[ 74-76 was announced on January 31, 1966, with a capacity of 3.913 MB. These were the only drums announced for the S/360 and S/370, and their niche was later filled by fixed-head disks.

The 6,000 RPM 2305 appeared in 1970, with capacities of 5 Mb (2305-1) or 11 Mb (2305-2) per module.Although these devices did not have large capacity, their speed and transfer rates made them attractive for high-performance needs. A typical use was overlay linkage (e.g. for OS and application subroutines) for program sections written to alternate in the same memory regions. Fixed head disks and drums were particularly effective as paging devices on the early virtual memory systems. The 2305, although often called a "drum" was actually a head-per-track disk device, with 12 recording surfaces and a data transfer rate up to 3 megabytes per second.

Rarely seen was the IBM 2321 Data Cell, a bizarre (and mechanically dramatic) device that contained multiple magnetic strips to hold data; strips could be randomly accessed, placed upon a cylinder-shaped drum for read/write operations; then returned to an internal storage cartridge. The IBM Data Cell [noodle picker] was among several IBM trademarked "speedy" mass online direct-access storage peripherals (reincarnated in recent years as "virtual tape" and automated tape librarian peripherals). The 2321 file had a capacity of 400 MB, at the time when the 2311 disk drive only had 7.2 MB. The IBM Data Cell was proposed to fill cost/capacity/speed gap between magnetic tapes—which had high capacity with relatively low cost per stored byte—and disks, which had higher expense per byte. Some installations also found the electromechanical operation less dependable and opted for less mechanical forms of direct-access storage.


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