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Originally published in Queue vol. 8, no. 5
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Bill Ryder | Sun, 08 Sep 2013 09:23:15 UTC

I like Howard L. Kaplan's explanation.

It's about a 7-8 ms from the bottom to the top of the slope which is in the right ballpark for a full disk rotation.

I remember the days of disk arrays where they would synchronise their spindle rotation to help deal with this.


Kellyn Pedersen | Tue, 19 Oct 2010 02:24:50 UTC

Id be interested to see the cost correlation via heat mapping when Oracle is able to process in memory, (PGA specific hash/sorts) vs. if performed on standard disk, (temp) vs. specialized disk such as FusionIO and others able to perform high capacity reads/writes.


Mike Meehan | Tue, 19 Oct 2010 01:53:36 UTC

Is the latency heatmap graphing library available? What are good tools for implementing this kind of visualization?


Howard L. Kaplan | Fri, 20 Aug 2010 21:06:33 UTC

I think I understand the behavior shown in figure 5. I think it's related to the beats caused by playing two similar-frequency sine waves or to the visual effects of Moire patterns.

The upward and downward slopes are always equal to each other, though the slopes themselves sometimes change. The test program writes to the two disks in strict alternation. If the rotational speeds of the two disks are slightly different, then the two disks' platter orientations will drift slowly with respect to each other. For a while, immediately after a write to disk 2, disk 1 will be in position to respond immediately. Over time, disk 1's position will become less and less optimal, leading to longer latencies, until it's suddenly just more than one full rotation behind the optimal point, in which case it can respond immediately again. As the latency to write to disk 1 increases, disk 2 becomes in a better position to respond immediately after each write to disk 1. Since each disk's latency is measured from the completion of the other disk's operation, we see the resulting "X" pattern. If the rotational speeds sometimes change slightly, that would cause changes in the slopes of the lines making up each "X".


Steve | Fri, 02 Jul 2010 09:04:26 UTC

Interesting article. Can you make the data available for analysis?


Brendan Gregg | Sun, 06 Jun 2010 00:17:25 UTC

Sorry about the image resolution, the PDF does look better; while the patterns are still visible, if you would like to look at the original screenshots they are http://blogs.sun.com/brendan/resource/analytics-5/figure1.png through figure9.png, and are linked here: http://blogs.sun.com/brendan/entry/visualizing_system_latency


Michael | Fri, 04 Jun 2010 20:09:13 UTC

@Ben I'm not sure what the problem is, I don't think anyone wouldn't presume that they display time series data - which they all are. As for the Y-axis obviously denotes a quantity that is in relation with subject at hand, scale is largely unimportant apart from being linear of which I'd also doubt anyone would presume otherwise.

Graphs are for patterns, trends - anyone who takes measurements from graphs should be taken out back to get shot.

The only reason you'd care about the actual time if you'd had to map it to a particular event/modification. but those are already pre-marked by a vertical line. The only reason you'd care about the scale of the quantified data would be if you actually had to compare it to an other system.


invisible | Thu, 03 Jun 2010 20:27:13 UTC

What tools were used to collect IO statistics and generate graphics?


Simon | Thu, 03 Jun 2010 12:34:18 UTC

Graphs looks pretty, but what do they say? I have no clue what I am looking at, article does not help me either.


S80Admin | Thu, 03 Jun 2010 11:40:04 UTC

Que cosa mas bonita polla!!!


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