rrdtool - round robin database tool

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rrdtool - | function



It is pretty easy to gather status information from all sorts of things, ranging from the temperature in your office to the number of octets which have passed through the FDDI interface of your router. But it is not so trivial to store this data in a efficient and systematic manner. This is where rrdtool kicks in. It lets you log and analyze the data you gather from all kinds of data-sources (DS). The data analysis part of rrdtool is based on the ability to quickly generate graphical representations of the data values collected over a definable time period.

In this man page you will find general information on the design and functionality of the Round Robin Database Tool (rrdtool). For a more detailed description of how to use the individual functions of the rrdtool check the corresponding man page.

For an introduction to the usage of rrdtool make sure you check the rrdtutorial manpage.


While the man pages talk of command line switches you have to set in order to make rrdtool work it is important to note that the rrdtool can be 'remote controlled' through a set of pipes. This saves a considerable amount of startup time when you plan to make rrdtool do a lot of things quickly. Check the section on Remote Control further down. There is also a number of language bindings for rrdtool which allow you to use it directly from perl, python, tcl, php, ...

Set up a new Round Robin Database (RRD). Check the rrdcreate manpage.

Store new data values into an RRD. Check the rrdupdate manpage.

Create a graph from data stored in one or several RRD. Apart from generating graphs, data can also be extracted to stdout. Check the rrdgraph manpage.

Dump the contents of an RRD in plain ASCII. In connection with restore you can use it to transport an rrd from one architecture to another. Check the rrddump manpage.

Restore an RRD in XML format to a binary rrd ... Check the rrdrestore manpage

Get data for a certain time period from a RRD. The graph function uses fetch to retrieve its data from an rrd. Check the rrdfetch manpage.

Alter setup of an RRD. Check the rrdtune manpage.

Find last update time of an RRD. Check the rrdlast manpage.

Display information about an RRD ... Check the rrdinfo manpage

Change the size of individual RRAs ... Dangerous! Check the rrdresize manpage.

Export data retrieved from one or several RRD. Check the rrdxport manpage

This is a standalone tool for producing rrd graphs on the fly. Check the rrdcgi manpage.


Data acquisition
When monitoring the state of a system, it is convenient to have the data available at a constant interval. Unfortunately you may not always be able to fetch data at exactly the time you want to. Therefore rrdtool lets you update the logfile at any time you want. It will automatically interpolate the value of the data-source (DS) at the latest official time-slot and write this value to the log. The value you have supplied is stored as well and is also taken into account when interpolating the next log entry.

You may log data at a 1 minute interval, but you are also be interested to know the development of the data over the last year. You could do this by simply storing the data in 1 minute interval, for one year. While this would take considerable disk space it would also take a lot of time to analyze the data when you wanted to create a graph covering the whole year. rrdtool offers a solution to this problem through its data consolidation feature. When setting up an Round Robin Database (RRD), you can define at which interval this consolidation should occur, and what consolidation function (CF) (average, minimum, maximum, last) should be used to build the consolidated values (see rrdcreate). You can define any number of different consolidation setups within one RRD. They will all be maintained on the fly when new data is loaded into the RRD.

Round Robin Archives
Data values of the same consolidation setup are stored into Round Robin Archives (RRA). This is a very efficient manner to store data for a certain amount of time, while using a known amount of storage space.

It works like this: If you want to store 1000 values in 5 minute interval, rrdtool will allocate space for 1000 data values and a header area. In the header it will store a pointer telling which one of the values in the storage area was last written to. New values are written to the Round Robin Archive in a ... you guess it ... round robin manner. This automatically limits the history to the last 1000 values. Because you can define several RRAs within a single RRD, you can setup another one, storing 750 data values at a 2 hour interval and thus keeping a log for the last two months although at a lower resolution.

The use of RRAs guarantees that the RRD does not grow over time and that old data is automatically eliminated. By using the consolidation feature, you can still keep data for a very long time, while gradually reducing the resolution of the data along the time axis. Using different consolidation functions (CF) allows you to store exactly the type of information that actually interests you. (Maximum one minute traffic on the LAN, minimum temperature of the wine cellar, total minutes down time ...)

Unknown Data
As mentioned earlier, the RRD stores data at a constant interval. Now it may happen that no new data is available when a value has to be written to the RRD. Data acquisition may not be possible for one reason or an other. The rrdtool handles these situations by storing an *UNKNOWN* value into the database. The value '*UNKNOWN*' is supported through all the functions of the database. When consolidating the amount of *UNKNOWN* data is accumulated and when a new consolidated value is ready to be written to its Round Robin Archive (RRA) a validity check is performed to make sure that the percentage of unknown data in the new value is below a configurable level. If so, an *UNKNOWN* value will be written to the RRA.

The rrdtool also allows one to generate reports in numerical and graphical form based on the data stored in one or several RRDs. The graphing feature is fully configurable. Size, color and contents of the graph can be defined freely. Check the rrdgraph manpage for more information on this.


When you start rrdtool with the command line option '-', it waits for input via standard in. With this feature you can improve performance by attaching rrdtool to another process (mrtg is one example) through a set of pipes. Over the pipes rrdtool accepts the same arguments as on the command line. When a command is completed, rrdtool will print the string 'OK', followed by timing information of the form u:usertime s:systemtime both values are running totals of seconds since rrdtool was started. If an error occurs, a line of the form 'ERROR: Description of error' will be printed. rrdtool will not abort if possible, but follow the ERROR line with an OK line.


rrdcreate, rrdupdate, rrdgraph, rrddump, rrdfetch, rrdtune, rrdlast, rrdxport


Bugs ? Features !


Tobias Oetiker <>