Adjusting Quantize Plots

The NFS latency statistic in the screen shot is rendered as a quantize plot. The name refers to the how the data is collected and displayed. For each statistic update, data is quantized into buckets, which are drawn as blocks on the plot. The more events in that bucket for that second, the darker the block will be drawn.

The example screen shot shows NFSv3 operations were spread out to 9 ms and beyond, with latency on the y-axis, until an event kicked in about halfway, and the latency dropped to less than 1 ms. Other statistics can be plotted to explain the drop in latency (the filesystem cache hit rate showed steady misses go to zero at this point: A workload had been randomly reading from disk (0 to 9+ ms latency), and switched to reading files that were cached in DRAM).

Quantize plots are used for I/O latency, I/O offset, and I/O size, and provide the following features:

• Detailed understanding of data profile (not just the average, maximum or minimum) these visualize all events and promote pattern identification.

• Vertical outlier elimination. Without this, the y-axis would always be compressed to include the highest event. Click on the crop outliers icon to toggle between different percentages of outlier elimination. Hover over this icon to see the current value.

• Vertical zoom: Click on a low point from the list in the left box, then shift-click on a high point. Now click on the crop outliers icon to zoom to this range.

• The height may be expanded. Look for a white line beneath in the middle of the plot, click on it, and drag downward.

• The width will expand to match the size of your browser.

• Click on and drag the move icon to switch the vertical location of the statistics.