IBM System/370 Manual page 54

A recording track in a Modell consists of two logical track segments.
One track segment occupies a 180-degree arc on the top surface of a
disk, while the other track segment occupies a 180-degree arc on the
bottom surface of the same disk, directly underneath the top segment.
TWo recording elements are paired to access each addressable track
in parallel. One element records data on the track segment on the
top of the disk surface while the other element records data on the
track segment on the bottom of the disk surface. Data is recorded
in parallel, two bits at a time, one bit on each segment. In addition,
all odd bytes are recorded on the top track segment. while all even
bytes are recorded on the bottom track segment. Half a rotation is
required to record a full track of data. All odd track addresses
occupy the same half of a disk, while all even track addresses occupy
the remaining half. There are 64 recording and 8 spare tracks on each
of the six disks in a module.
Conceptually, the 2305 Model 1 can
be
thought to contain two access
mechanisms 180 degrees apart, each of which has 36 pairs of recording
elements per disk, as shown in Figure 20.15.2. (In reality there are
two pairs of access mechanisms.) Each of the addressable tracks in
one module (384 recording and 48 spare) can be accessed
by
both
mechanisms. The 2835 control unit dynamically selects the mechanism
closest to the desired record when a command is issued. The maximum
rotational delay is, therefore. half a track rotation (5 ms) and the
average delay is 2.5 ms. (See Figure 20.15.2.)
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