【Fujitsu】 M2915 Magnetic Disk Unit

The M2915 was a magnetic disk unit intended for use with servers that featured a cutting-edge magneto-resistive (MR) head. Fujitsu began shipping the unit in September 1994. At the time, most server-class magnetic disk units were based on five-inch drives, but Fujitsu developed the M2915 as one of the first 3.5-inch units for servers. Following this model, Fujitsu rebranded its 3.5-inch enterprise magnetic disk units the Allegro series and grabbed a solid position in the OEM market.

By combining the newly developed MR head with a new signal processing method, PR4ML,*1 the M2915 realized an area recording density of 250 Mbits per square inch (71,627 bytes per inch and 3,553 tracks per inch), which at the time was among the highest on the market, for a total storage capacity of 2.1 GB. Fujitsu also achieved a disk speed of 7,200 rpm (compared to 5,400 rpm for the M2903 and M2909 models from the same series) and an average access time of just 22 milliseconds.

Alongside the introduction of an MR head, Fujitsu’s early development of cable patterned suspension (CAPS) deserves special mention. CAPS ran the read line wiring to the MR head along the support spring to isolate the read line from external forces and to stabilize the suspended head. Although MR heads had more terminals than conventional heads, they had been wired with conventional stranded conductors. Beginning with this model in 1993, Fujitsu used CAPS technology on all its magnetic disk units.

*1. PR4ML is a signal processing method. Using partial responses (PR), it removes waveform interference and cancels noise by limiting bandwidth. It detects the maximum-likelihood (ML) signal for every two-bit pair, not each bit, using the Viterbi algorithm.

M2915 magnetic disk unit (photo shows the 3.5-inch MR head)