If you know MRTG, you can think of RRDtool as a reimplementation of
MRTGs graphing and logging features. Magnitudes faster and more flexible
than you ever thought possible

RRD is the Acronym for Round Robin Database. RRD is a system to store
and display time-series data (i.e. network bandwidth, machine-room
temperature, server load average). It stores the data in a very compact
way that will not expand over time, and it presents useful graphs by
processing the data to enforce a certain data density. It can be used
either via simple wrapper scripts (from shell or Perl) or via frontends
that poll network devices and put a friendly user interface on it

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