IBM System/370 145 Manual page 224

I/O requests can be initiated in a given period of time, since
the channel is free more often. When many disk devices are
active concurrently on a block multiplexer channel, there is more
potential for such overlap.
2. Direct access devices with RPS should be placed on separate
channels from I/O devices without RPS. Alternatives are as
foll.ows:
a. If it is necessary to place non-RPS devices on the same block
multiplexer channel with RPS devices, give first choice to
non-RPS devices with a buffered control unit, such as the
3505 Card Reader, the 3525 Card Punch and the 3211 Printer.
These devices disconnect from a block multiplexer channel
during the relativel.y long mechanical portion of their cycle,
thereby freeing the channel for other operations .•
b. Tape units should not be placed on a block multiplexer
channel with RPS devices unless absolQtely necessary, because
channel disconnection does not occur during any of their
channel operations. If this is not possible, try to plan job
scheduling to avoid having jobs using tape units and jobs
using RPS support active on a block multiplexer channel at
the same time. If this is not feasible, try to assign very
low-activity data sets to these
tqpe
units.
A device without channel disconnect capability can monopolize the
block multiplexer channel for relatively long periods of time,
thereby preventing
(1)
the initiation of other I/O operations on
the channel and (2) the reconnect ion and completion of disk RPS
channel programs already in operation on the channel. For
example, a direct access device without RPS retains use of the
channel during its search operations as well as during its reads
and writes. If the device is a 2314 and block size is half a
track, the channel is busy for 25 ms on the average (12.5
ms
average rota'tional delay plus 12.5 ms read/wri tel for each I/O
operation started for the non-RPS 231q facility. Even if the
block size used is relatively small, the channel can still be
monopolized by the non-RPS device if there is high activity on
the device.
3. The 2305 facility normally should not be placed on a block
multiplexer channel with any other device.
Exclusive use of a channel ensures optimum performance of the
2305 facility as a system residence device.
The following should be noted as regards specification of priority
and ordered-seek I/O request queuing options for RPS devices at OS
system generation. The priority queuing option ensures priority I/O
request initiation for the device, but because of first-come, first-
served handling of I/O ope+ations on the block multiplexer channel, this
option does not ensure that priority device channel programs will
complete sooner than other RPS channel programs that were started later
on the channel. However, the objective of specifying the ordered-seek
queuing option (minimization of arm movement on a disk drive> can still
be achieved when using RPS.
JOB SCHEDULING
Guidelines for job scheduling on the System/310 Model lq5 are:
1. If total system throughput improvement, rather than maximum
individual job performance increase, is the objective, schedule
21q A Guide to the IBM System/310 Model 145