Expressions example: Read IOPS percentage

Using expressions, you can view metrics by alternate means, such as percentage of total.

About this task

In this example we want to show Read IOPS as a percentage of Total IOPS. You can think of this as the following formula:

  • Read Percentage = (Read IOPS / Total IOPS) x 100

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

Steps

  1. Create a new dashboard, or open an existing dashboard in edit mode.
  2. Add a widget to the dashboard. Choose Area 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.
  3. Click the Convert to Expression button.
    The current query is converted to Expression mode. Notice that you cannot change the asset type while in Expression mode. Notice that while you are in Expression mode, the button changes to Revert to Query. Click this 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.

  4. The IOPS - Total metric is now in the alphabetic variable field "a". In the "b" variable field, click Select and choose IOPS - Read.
    You can add up to a total of five alphabetic variables for your expression by clicking the + button following the variable fields. For our Read Percentage example, we only need Total IOPS ("a") and Read IOPS ("b").
  5. In the Expression field, you use the letters corresponding to each variable to build your expression. We know that Read Percentage = (Read IOPS / Total IOPS) x 100, so we would write this expression as: (b / a) * 100
  6. The Label field identifies the expression. Change the label to "Read Percentage", or something equally meaningful for you.
  7. Change the Units field to "%" or "Percent".
    The chart displays the IOPS Read percentage over time for the chosen storage devices. If desired, you can set a filter, or choose a different rollup method. Be aware that if you select Sum as the rollup method, all percentage values are added together, which potentially may go higher than 100%.
  8. Click Save to save the chart to your dashboard.

    You can also use expressions in Line chart, Spline chart, or Stacked Area chart widgets.