Chapter 7. Sysplex functions 183
Value of Channel Subsystem Priority Queueing
The benefits proved by Channel Subsystem Priority Queueing include the following:
򐂰 Improved performance
I/O from work that is not meeting its goals may be given priority over I/O from work that is
meeting its goals, providing Workload Manager with an additional method for adjusting I/O
performance. Channel Subsystem Priority Queueing is complementary to UCB priority
queueing and control unit priority queueing, each addressing a different queueing
mechanism that may affect I/O performance.
򐂰 Reduced skills required to manage z/OS
Monitoring and tuning requirements are reduced because of the self-tuning abilities of the
Channel Subsystem.
7.5.4 WLM and Channel Subsystem priority
WLM assigns the highest to lowest CSS priority, as given in Table 7-5. It assigns eight priority
levels.
Table 7-5 WLM-assigned CSS I/O priorities
Work that is meeting its WLM target is assigned CSS priorities between F9 and FC,
depending on its execution profile. Work that has a light I/O usage has its CSS priority moved
upwards.
When an I/O operation is started by a CP on the Server, it can be queued by the Channel
Subsystem for several reasons, including Switch port busy, Control unit busy, Device busy,
and All channel paths busy. Queued I/O requests are started or restarted when an I/O
completes or the Control unit indicates the condition has cleared. Where two or more I/O
requests are queued in the Channel Subsystem, the CSS LIC on the zSeries selects the
requests in priority order. The LIC also ages requests to ensure that low priority requests are
not queued for excessive periods.
In the LPAR image profile for the z/OS image, there are two specifications that relate to the
Channel Subsystem I/O Priority Queueing. They are:
򐂰 The range of priorities that will be used by this image
򐂰 The default Channel Subsystem I/O priority
For images running operating systems that do not support Channel Subsystem priority,
the customer can prioritize all the Channel Subsystem requests coming from that image
against the other images by specifying a value for the default priority.
Within an LPAR cluster, the prioritization is managed by WLM goal mode and coordinated
across the cluster. Hence, the range should be set identically for all logical partitions in the
same LPAR cluster.
Workload type Priority
System work. FF
Importance of 1 and 2 missing goals. FE
Importance of 3 and 4 missing goals. FD
Meeting goals. Adjust by ratio of connect time to elapsed time. F9-FC
Discretionary. F8

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