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TNL GN20-2277
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 231q and block size is half a track, the channel is busy
for 25 ms on the average (12.5 ms average rotational delay
plus 12.5 ms read/write) 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 other devices.
Exclusive use of a channel insures optimum performance of the
2305 facility as a system residence device.
The following should be noted in regard to specification of priority
and ordered-seek I/O request queuing options for RPS devices at OS
system generation.
The priority queuing option insures priority I/O
request initiation for the device, but because of first-corne, first-
served handling of I/O operations on the block multiplexer channel,
this option does not insure 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 RPS is used.
90.1
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