IBM System/370 Guide

Page of GC20-1730-0
Revised 11/20/70
By TNL GN20-2277
Tapes that cannot be converted on an as-used basis, such as
program tapes or active reference tapes that are not rewritten when
processed, can be converted by using a copy utility.
60:10 PLANNING OPTI~illL SYSTEM PERFORMANCE, USING BLOCK MULTIPLEXER
CHANNELS AND RPS DEVICES
Block multiplexing, rotational position sensing, and multiple
requesting provide the user with another tool that can improve total
system throughput. However, the effectiveness of this tool for a given
installation depends largely on proper planning for its use. This
section indicates how to use block multiplexer channels and RPS devices
effectively.
The guidelines outlined indicate how best to configure a system
with rotational position sensing devices, how to plan job scheduling,
and what to consider when determining disk data set characteristics.
Explanations follow the statement of each guideline.
All guidelines presented are not necessarily practical for all
users. Each item should be evaluated in terms of the processing
requirements and hardware configuration of an installation.
SYSTEM CONFIGURATION AND GENERATION
90
System configuration and generation guidelines are as follows:
1. Multiple 3330 facilities should be placed on a single block
multiplexer channel.
Performance improvement occurs (1) as a result of overlapping
the rotational positioning time of disk devices and (2) because
more 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. Alternate possibilities
are as follows:
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 2540 Card Read Punch and the 1403 Printer:
These devices disconnect from a block multiplexer channel
during the relatively 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 absolutely 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 lo~activity data sets to these tape 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 reconnection and
completion of disk RPS channel programs already in operation

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English