Expressions example: "System" I/O

Expressions give you the freedom to chart data that can be calculated from other metrics.

About this task

Example 2: OnCommand Insight acquires many metrics from data sources. Among them are read, write, and total IOPS. However, the total number of IOPS reported by acquisition sometimes includes "system" IOPS, which are those IO operations that are not a direct part of data reading or writing. This system I/O can also be thought of as "overhead" I/O, necessary for proper system operation but not directly related to data operations.

To show these system I/Os, you can subtract read and write IOPS from the total IOPS reported from acquisition. The formula might look like this:

System IOPS = Total IOPS - (Read IOPS + Write IOPS)

This data can then be shown in a line graph on your dashboard. To do this, follow these steps:

Procedure

Create a new dashboard, or open an existing dashboard in edit mode.
Add a widget to the dashboard. Choose Line chart.
The widget opens in edit mode. By default, a query is displayed showing IOPS - Total for Storage assets. If desired, select a different asset type.
Click the button to create a copy of the query.
A duplicate of the query is added below the original.
In the second query, click the Convert to Expression button.
The current query is converted to Expression mode. Click Revert to Query if you wish to switch back to Query mode at any time. Be aware that switching between modes will reset fields to their defaults.
For now, stay in Expression mode.
The IOPS - Total metric is now in the alphabetic variable field a. Click on IOPS - Total and change it to IOPS - Read. .
In the b variable field, click Select and choose IOPS - Write.
In the Expression field, you use the letters corresponding to each variable to build your expression. We would write our expression simply as: a + b. In the Display section, choose Area chart for this expression.
The Label field identifies the expression. Change the label to System IOPS, or something equally meaningful for you.
The chart displays the total IOPS as a line chart, with an area chart showing the combination of read and write IOPS below that. The gap between the two shows the IOPS that are not directly related to data read or write operations.
Click Save to save the chart to your dashboard.