Selecting A Time Period For Analysis; Checking The Working Set Metric - HP Surestore Disk Array 12h - And FC60 System Administrator Manual

Using the ARM Command Line Utilities for MPE
Analyzing Disk Array Performance

Selecting a Time Period for Analysis

When analyzing performance, you must define the time period over which the analysis will be performed. A
starting time (stime) and ending time (etime) establish the analysis period. For the best results, select a
time period when performance may be a concern. This will produce the most meaningful analysis and
recommendations.
For example, if the heaviest load on the disk array occurs between the hours of 8:00 AM and 5:00 PM,
restrict the analysis to this time period. If you include periods of less activity, the analysis may yield
different results and consequently different recommendations. This occurs because activity is averaged over
the entire analysis period, and periods of less activity will offset the effects of busier periods.
Typically, you should select a period of time that represents normal system operation. Avoid any unusual
events such as a Rebuild or changes made to array capacity. If you select a time period that includes an
event that may distort the analysis, the utility will alert you and will not provide any recommendations.
You can also control the display interval using the int option. This allows you to control how much detail
you get when displaying the metrics. The display interval is the number of 15-minute increments.

Checking the Working Set Metric

A key factor in monitoring and maintaining optimal performance of the disk array is the Working Set
metric. To ensure that disk array performance is maintained, you should access the performance metrics
regularly and check the Working Set value.
The Working Set performance metric is derived from the Write Working Set parameter. It indicates the
ratio of the Write Working Set size to the amount of RAID 0/1 space available. For a detailed explanation
of the Write Working Set and its impact on performance, refer to the HP SureStore E Disk Array 12H
User's and Service Manual.
To maintain performance, the amount of RAID 0/1 space should equal or exceed the Write Working Set,
resulting in a Working Set value less than or equal to 1. A Working Set value greater than 1 indicates that
the Write Working Set is larger than the available RAID 0/1 space and the disk array is servicing writes
from RAID 5 space.
If the Working Set consistently exceeds 1, the amount of RAID 0 /1 space available should be increased to
improve performance. This can be accomplished in several ways as described in the following section.
If the Working Set is consistently much less than 1, some of the RAID 0/1 capacity can be allocated to a
new LUN without impacting performance. The remaining RAID 0/1 space should be adequate to
accommodate the Write Working Set.
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