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From Andrew Bialecki <>
Subject determining the cause of a high CPU / disk util node
Date Sun, 03 Sep 2017 00:47:56 GMT
We're running Cassandra 3.7 on AWS, different AZs, same region. The columns
are counters and the workload is 95% writes, but of course those involves a
local read and write because their coutners.

We have a node with much higher CPU load than others under heavy write
volume. That node is at 100% disk utilization / high iowait. The IO load
when looked at with iostat is primarily reads (95%) vs writes in terms of
requests and bytes. Below's a graph of the CPU.

Any ideas to how we could diagnose what is causing so much IO vs. other

Also, we're not sure why this node in particular is hot the other two
"replica" nodes (we use RF = 3). We're using the DataStax driver and are
looking into the load balancing policy to see if that's an issue.

[image: Inline image 1]

Andrew Bialecki

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