Disk Arm Graphs
Disk arm utilization is described as a percent used of the time. The utilization is calculated as an
average over a long period of time, one hour or more. Acceptable utilization is less than 10% for
response time critical environments. This utilization is also the recommendation for new
installations or upgrades.
A value of 30% to 40% is considered marginal and depends on customer expectations. A value
above 40% is considered critical utilization. The recommended guideline is for a single arm in a
short time period, less than five minutes.
An abnormally high disk arm activity leads to an increase in waiting time to access the requested
data. This can mean longer response times and longer turnaround time for batch jobs.
High disk activity can be a result of one or all of the following conditions:
The amount of disk arms is insufficient for the workload
There is insufficient memory, resulting in excessive faulting
Disks are fragmented
o
Data is placed on disks in smaller and smaller segments as disk fragmentation
increases. When disks are fragmented, data is retrieved from several places
when a block of records is requested. This takes time and keeps the disk arms
busy.
The size of the disk drives
o
You can have sufficient disk space but an insufficient number of disk arms.
Consider a smaller GB per arm configuration if the data requires high availability
(frequent random access).
Lots of logical views with immediate maintenance
Consider adding disk arms on your partition if the disk arm utilization approaches or exceeds the
acceptable value. Having a partition with not enough memory and a high paging rate usually
causes high disk arm utilization. You must examine both memory and disk arm utilization to
understand the necessary action to take.
Three types of disk arm graphs are available in PM for Power Systems in support of IBM i as
explained in the following sections:
Peak Disk Arm Utilization in % graph
Disk Arm Utilization Monthly Average by Hour graph
Peak Disk Arm Utilization in %, 3 Month Trend graph
What to Look For in Disk Arm Graphs
Consider the following when reviewing disk arm graphs:
Is the utilization close to the guidelines?
Is the trend a rising one?
Is the utilization high all the time?
PM for Power Systems Graph Reference Document
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