The first disk drives for System/360 were IBM 2302s and IBM 2311s. The first drum for System/360 was the IBM 7320. 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. (When used with a 360/20, the 1316 pack was formatted into fixed-length 270 byte sectors, giving a maximum capacity of 5.4MB.) 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 was 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 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 System/360 I/O software could customize the length of the data record (variable-length records), as was the case with magnetic tapes. Some of the most powerful early System/360s used high-speed head-per-track drum storage devices. The 3,500 RPM 2301, which replaced the 7320, was part of the original System/360 announcement, with a capacity of 4 MB. The 303.8 KB/second IBM 2303 was announced on January 31, 1966, with a capacity of 3.913 MB. These were the only drums announced for System/360 and System/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