IBM 990 - Manual (Page 194)

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zSeries 990 Technical Guide
Simplified I/O definition
The connection between managed channels and managed control units does not have to
be explicitly defined.
Reduced skills required to manage z/OS
Managed channels and control units are automatically monitored, balanced, tuned, and
reconfigured.
Enhanced availability
A failing or hung channel path will result in reduced throughput on the affected control unit.
DCM will rapidly detect the symptom and augment the paths, automatically bypassing the
problem. The problem will still have to be analyzed and corrected by site personnel.
DCM will automatically analyze and minimize single points of failure on an I/O path by
selecting appropriate paths. DCM is sensitive to single points of failure, such as:
– ESCON or FICON channel cards
– I/O CHA cards
– Processor Self-Timed Interconnect
– Director port cards
– Control Unit I/O bay
– Control Unit Interface card
– ESCON Director
7.5.3 Channel Subsystem Priority Queueing
Channel Subsystem (CSS) Priority Queueing is a new function available on zSeries
processors in either
1
basic or LPAR mode. It allows the z/OS operating system to specify a
priority value when starting an I/O request. When there is contention causing queueing in the
Channel Subsystem, the request is prioritized by this value.
If important work is missing its goals due to I/O contention on channels shared with other
work, it will be given a higher Channel Subsystem I/O priority than the less important work.
This function goes hand in hand with the Dynamic Channel Path Management described
previously: as additional channel paths are moved to control units to help an important
workload meet goals, Channel Subsystem Priority Queuing ensures that the important work
receives greater access to additional bandwidth than less important work that happen to be
using the same channel.
Channel Subsystem Priority Queuing runs on a zSeries server in z/Architecture mode, in both
basic and LPAR mode. The participating z/OS system images can be defined as XCFLOCAL,
MONOPLEX, or MULTISYSTEM. It is optimized when WLM is running in goal mode. It does
not require a Coupling Facility structure.
Enabling Channel Subsystem Priority Queuing involves defining a range of I/O priorities for
each logical partition on the hardware management console, and then turning on the “Global
input/output (I/O) priority queuing” switch. (You also need to specify “YES” for WLM's I/O
priority management setting.)
z/OS will set the priority based on a goal mode WLM policy. This complements the goal mode
priority management that sets I/O priority for IOS UCB queues, and for queueing in the 2105
ESS disk subsystem.
CSS Priority Queueing uses different priorities calculated in a different way from the I/O
priorities used for UCB and control unit queueing.
1 The z990 operates in LPAR mode only