IBM 2305

The IBM 2305 was a pioneering fixed-head magnetic disk storage device introduced by International Business Machines (IBM) in 1970. This revolutionary storage solution represented a significant advancement in direct-access storage device (DASD) technology during the era of mainframe computer systems.

Development and Technical Specifications

Developed as part of IBM’s Storage Product Division , the 2305 emerged from IBM’s continuous innovation in magnetic disk storage technology following their groundbreaking IBM 350 disk storage unit. The device featured:

Fixed-head design: Unlike conventional moving-head disk drives, the 2305 utilized fixed read/write heads positioned over each track
5 million byte capacity: Storing approximately 4.7 MB of data (equivalent to about 5,000,000 characters)
High-speed access: Achieved access times of 2.5 milliseconds due to its fixed-head architecture
Data transfer rate: Capable of 1.2 million characters per second
Physical dimensions: Contained in a cabinet measuring 29.5" wide × 48" high × 31.5" deep

The 2305’s cylindrical storage medium rotated at 3,000 RPM, with data organized into 200 tracks per surface. Its non-volatile storage capability made it particularly valuable for critical system operations.

Implementation and Usage

The IBM 2305 found primary application with IBM’s flagship System/370 mainframe computers, where it served crucial roles:

Paging device: Functioned as high-speed virtual memory storage for operating systems like OS/VS1 and OS/VS2
System residence device: Stored frequently-accessed operating system components
Specialized storage: Employed in real-time processing applications requiring rapid data access

The device connected through the IBM 2880 Block Multiplexer Channel and required precise environmental controls, operating within 64-90°F temperature ranges.

Evolution and Legacy

The 2305 existed in two distinct models:

2305-1: Featured a single storage module
2305-2: Contained two storage modules with independent access mechanisms

This storage system preceded IBM’s subsequent innovations including the IBM 2304 (1965) and the influential IBM 3330 (1970), which introduced removable disk packs. The 2305’s fixed-head technology directly influenced the development of later cache memory systems and solid-state storage solutions.

While production ceased in 1975, the 2305 established critical architectural principles that shaped modern storage area networks and enterprise storage systems. Its combination of high-speed access and reliable data retention represented a milestone in the evolution from magnetic core memory to contemporary flash storage technologies.