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Tri avatar image
Tri asked ·

In the DS210 quiz, how does dstat help make sure Linux is not paging?

DS210 Linux dstat command Quiz

I made lengthy practice on dstat. And yet I failed this practice question again today.

"The linux dstat command helps you make sure your linux cluster is not paging"

The correct answer is: TRUE. How can that be? I would say FALSE would be a more appropriate answer.

1) dstat displays information. It is not designed to stop paging activity.

2) dstat is not cluster aware. And how would it care you have a Cassandra cluster? You run dstat on a NODE. And somehow you expect it to stop paging on the CLUSTER ?!?!

academyds210linux
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1 Answer

Erick Ramirez avatar image
Erick Ramirez answered ·

Is it possible that you are perhaps confusing the word paging as the CQL query paging?

For the purposes of the DS210 course and the dstat unit, it refers to the Linux memory paging in and out. It doesn't refer to CQL query results getting paged by the driver.

The dstat utility helps you identify memory issues when the paging statistics are high. This is the reason the answer to the quiz question is TRUE. Cheers!

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dstat is linux and has nothing to do with CQL. The paging here is related to the OS, not having enough RAM and spilling over to saving on disk on the swap device.

The issue I report here is mostly about the ambiguous wording which could be interpreted subjectively. Similar to asking "Do you think you are hungry?" Any answer is good. I would suggest to change the wording of the question.

Instead of:

The linux dstat command helps you make sure your linux cluster is not paging

Maybe change to:

The linux dstat command helps you to detect swap events on your linux node

The most notable change here is to imply dstat is a tool to help you see a problem. Not to stop it. And also remove the word cluster from the question. In case you think this is not correct, can you please show an example of using dstat on a node to stop paging (swapping) on a different node located somewhere in the cluster?

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It feels like we're splitting hairs here. But thanks for the feedback. Cheers!

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BTW The solution is well explained in DS210 Tuning the Kernel Which could be summarized as "It is better to have a node go down rather than limping along with swapping". See course for ways to turn swap off.

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