final book al hoagland - Archive Server - Computer History Museum

CHAPTER 4
The ADF (Advanced Disk File) and Future Generation Disk Drives
While the magnetic disk drive clearly demonstrated significant capacity improvements
for data storage, it still did not offer either the access time or the capacity needed to
implement the kind of rapid transaction processing needed for many computer
applications. Companies such as UNIVAC were therefore aggressively developing high
performance magnetic drums for mass data storage where access times superior to disks
had been demonstrated. Given this environment, it is fair to say that a more advanced
successor to the magnetic disk drive was viewed within IBM as absolutely critical to
ensure magnetic disk drives were the future direction for data storage and that IBM had
chosen the right path by focusing on disk drives and not drums or other technologies.
Even before the announcement of the RAMAC disk drive in 1956, IBM had made a
decision in 1955 to move ahead with the design and development of the next generation
disk drive, with a project named ADF (Advanced Disk File). The goals of the ADF
project were to greatly surpass the capacity of the RAMAC while dramatically shortening
access times One of the first new commercial applications targeted for this next-
generation IBM disk drive was the new American Airlines Airline Reservations System
(Sabre), which depended on a geographically dispersed real time on-line transaction
processing capability. IBM was also developing a new supercomputer, called Stretch, in
order to gain the lead in scientific computing. This computer would also require a disk
drive with similar advances in capacity and access time.
When the RAMAC project became a formal product development program in 1955, it
was placed under Lou Stevens and transferred to a new manufacturing organization
associated with a new IBM plant site in San Jose. The overall future of the disk drive
became totally vested in the ADF program under Rey Johnson and his original research
laboratory (at 99 Notre Dame) was moved to a larger facility in a dilapidated warehouse
several blocks away. The ADF project under Rey Johnson began, somewhat like the
original RAMAC project, by first choosing advanced performance goals to meet the new
market opportunities, since there was no advanced research work to draw upon.
Essentially, two performance goals set the technology objectives for the ADF. These
were: a faster access to data by almost a factor of 10 and an increase in data capacity 10
times that of the RAMAC with the same size disk stack.
A major issue to address in improving access times for the ADF project was the question
of head design. Heads are the mechanisms that read data from or write data to a magnetic
disk or tape. The magnetic read/write head is imbedded in a small metallic carrier, called
a slider, which is contoured like an airfoil to create and maintain the spacing of the head
from the disk. The RAMAC had two heads, one to read/write from the top surface of a
disk and one to read/write from the bottom surface. This meant that the heads had to be
repositioned up and down the disk stack each time in order to access the correct disk and
track and therefore access time was slow. To decrease access time, the solution was to
have a head positioned to read/write each disk individually so no repositioning was
necessary. To increase capacity it was necessary to decrease the head-to-disk spacing.
18