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Linux performance basics

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Linux

I want to write about Cassandra performance tuning, but first I need to cover some basics: how to use vmstat, iostat, and top to understand what part of your system is the bottleneck -- not just for Cassandra but for any system.

vmstat

You will typically run vmstat with "vmstat sampling-period", e.g., "vmstat 5." The output looks like this:

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
20 0 195540 32772 6952 576752 0 0 11 12 38 43 1 0 99 0
22 2 195536 35988 6680 575132 6 0 2952 14 959 16375 72 21 4 3

The first line is your total system average since boot; typically this will not be very useful, since you are interested in what is causing problems NOW. Then you will get one line per sample period; most of the output is self explanatory. The reason to start with vmstat is the "swap" section: si and so are swap in (memory read from disk) and swap out (memory written to disk).

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