The documentation provided with RRDtool can be too technical for some people. This tutorial is here to help you understand the basics of RRDtool. It should prepare you to read the documentation yourself. It also explains the general things about statistics with a focus on networking.
Sometimes things change. This example used to provide numbers like ``0.04'' in stead of ``4.00000e-02''. Those are really the same numbers, just written down differently. Don't be alarmed if a future version of rrdtool displays a slightly different form of output. The examples in this document are correct for version 1.2.0 of RRDtool.
Also, sometimes bugs do occur. They may also influence the outcome of the examples. Example speed4.png was suffering from this (the handling of unknown data in an if-statement was wrong). Normal data will be just fine (a bug in rrdtool wouldn't last long) but special cases like NaN, INF and so on may last a bit longer. Try another version if you can, or just live with it.
I fixed the speed4.png example (and added a note). There may be other examples which suffer from the same or a similar bug. Try to fix it yourself, which is a great excercise. But please do not submit your result as a fix to the source of this document. Discuss it on the user's list, or write to me.
Many examples below talk about SNMP which is an acronym for Simple Network Management Protocol. ``Simple'' refers to the protocol. It does not mean it is simple to manage or monitor a network. After working your way through this document, you should know enough to be able to understand what people are talking about. For now, just realize that SNMP can be used to query devices for the values of counters they keep. It is the value from those counters that we want to store in the RRD.
Most likely you will start to use RRDtool to store and process data collected via SNMP. The data will most likely be bytes (or bits) transfered from and to a network or a computer. But it can also be used to display tidal waves, solar radiation, power consumption, number of visitors at an exhibition, noise levels near an airport, temperature on your favorite holiday location, temperature in the fridge and whatever you imagination can come up with.
You only need a sensor to measure the data and be able to feed the numbers into RRDtool. RRDtool then lets you create a database, store data in it, retrieve that data and create graphs in PNG format for display on a web browser. Those PNG images are dependent on the data you collected and could be, for instance, an overview of the average network usage, or the peaks that occurred.
If on the other hand the program works but does not give you the expected results, it will be a problem with configuring it. Review your configuration and compare it with the examples that follow.
There is a mailing list and an archive of it. Read the list for a few weeks and search the archive. It is considered rude to just ask a question without searching the archives: your problem may already have been solved for somebody else! This is true for most, if not all, mailing lists and not only for this particular one. Look in the documentation that came with RRDtool for the location and usage of the list.
I suggest you take a moment to subscribe to the mailing list right now by sending an email to <firstname.lastname@example.org> with a subject of ``subscribe''. If you ever want to leave this list, just write an email to the same address but now with a subject of ``unsubscribe''.
By following the examples you get some hands-on experience and, even more important, some background information of how it works.
You will need to know something about hexadecimal numbers. If you don't then start with reading bin_dec_hex before you continue here.
We will start with some easy stuff and compare a car with a router, or compare kilometers (miles if you wish) with bits and bytes. It's all the same: some number over some time.
Assume we have a device that transfers bytes to and from the Internet. This device keeps a counter that starts at zero when it is turned on, increasing with every byte that is transfered. This counter will probably have a maximum value. If this value is reached and an extra byte is counted, the counter starts over at zero. This is the same as many counters in the world such as the mileage counter in a car.
Most discussions about networking talk about bits per second so lets get used to that right away. Assume a byte is eight bits and start to think in bits not bytes. The counter, however, still counts bytes! In the SNMP world most of the counters are 32 bits. That means they are counting from 0 to 4294967295. We will use these values in the examples. The device, when asked, returns the current value of the counter. We know the time that has passes since we last asked so we now know how many bytes have been transfered ***on average*** per second. This is not very hard to calculate. First in words, then in calculations:
bps = (counter_now - counter_before) / (time_now - time_before) * 8
For some people it may help to translate this to an automobile example. Do not try this example, and if you do, don't blame me for the results!
People who are not used to think in kilometers per hour can translate most into miles per hour by dividing km by 1.6 (close enough). I will use the following abbreviations:
m: meter km: kilometer (= 1000 meters). h: hour s: second km/h: kilometers per hour m/s: meters per second
You are driving a car. At 12:05 you read the counter in the dashboard and it tells you that the car has moved 12345 km until that moment. At 12:10 you look again, it reads 12357 km. This means you have traveled 12 km in five minutes. A scientist would translate that into meters per second and this makes a nice comparison toward the problem of (bytes per five minutes) versus (bits per second).
We traveled 12 kilometers which is 12000 meters. We did that in five minutes or 300 seconds. Our speed is 12000m / 300s or 40 m/s.
We could also calculate the speed in km/h: 12 times 5 minutes is an hour, so we have to multiply 12 km by 12 to get 144 km/h. For our native English speaking friends: that's 90 mph so don't try this example at home or where I live :)
Remember: these numbers are averages only. There is no way to figure out from the numbers, if you drove at a constant speed. There is an example later on in this tutorial that explains this.
I hope you understand that there is no difference in calculating m/s or bps; only the way we collect the data is different. Even the k from kilo is the same as in networking terms k also means 1000.
We will now create a database where we can keep all these interesting numbers. The method used to start the program may differ slightly from OS to OS, but I assume you can figure it out if it works different on your's. Make sure you do not overwrite any file on your system when executing the following command and type the whole line as one long line (I had to split it for readability) and skip all of the '\' characters.
rrdtool create test.rrd \ --start 920804400 \ DS:speed:COUNTER:600:U:U \ RRA:AVERAGE:0.5:1:24 \ RRA:AVERAGE:0.5:6:10
(So enter: "rrdtool create test.rrd --start 920804400 DS ...")
RRDtool works with special time stamps coming from the UNIX world. This time stamp is the number of seconds that passed since January 1st 1970 UTC. The time stamp value is translated into local time and it will therefore look different for different time zones.
Chances are that you are not in the same part of the world as I am. This means your time zone is different. In all examples where I talk about time, the hours may be wrong for you. This has little effect on the results of the examples, just correct the hours while reading. As an example: where I will see ``12:05'' the UK folks will see ``11:05''.
We now have to fill our database with some numbers. We'll pretend to have read the following numbers:
12:05 12345 km 12:10 12357 km 12:15 12363 km 12:20 12363 km 12:25 12363 km 12:30 12373 km 12:35 12383 km 12:40 12393 km 12:45 12399 km 12:50 12405 km 12:55 12411 km 13:00 12415 km 13:05 12420 km 13:10 12422 km 13:15 12423 km
We fill the database as follows:
rrdtool update test.rrd 920804700:12345 920805000:12357 920805300:12363 rrdtool update test.rrd 920805600:12363 920805900:12363 920806200:12373 rrdtool update test.rrd 920806500:12383 920806800:12393 920807100:12399 rrdtool update test.rrd 920807400:12405 920807700:12411 920808000:12415 rrdtool update test.rrd 920808300:12420 920808600:12422 920808900:12423
This reads: update our test database with the following numbers
time 920804700, value 12345 time 920805000, value 12357
As you can see, it is possible to feed more than one value into the database in one command. I had to stop at three for readability but the real maximum per line is OS dependent.
We can now retrieve the data from our database using ``rrdtool fetch'':
rrdtool fetch test.rrd AVERAGE --start 920804400 --end 920809200
It should return the following output:
speed 920804700: nan 920805000: 4.0000000000e-02 920805300: 2.0000000000e-02 920805600: 0.0000000000e+00 920805900: 0.0000000000e+00 920806200: 3.3333333333e-02 920806500: 3.3333333333e-02 920806800: 3.3333333333e-02 920807100: 2.0000000000e-02 920807400: 2.0000000000e-02 920807700: 2.0000000000e-02 920808000: 1.3333333333e-02 920808300: 1.6666666667e-02 920808600: 6.6666666667e-03 920808900: 3.3333333333e-03 920809200: nan
If it doesn't, something may be wrong. Perhaps your OS will print ``NaN'' in a different form. ``NaN'' stands for ``Not A Number''. If your OS writes ``U'' or ``UNKN'' or something similar that's okay. If something else is wrong, it will probably be due to an error you made (assuming that my tutorial is correct of course :-). In that case: delete the database and try again.
The meaning of the above output will become clear below.
rrdtool graph speed.png \ --start 920804400 --end 920808000 \ DEF:myspeed=test.rrd:speed:AVERAGE \ LINE2:myspeed#FF0000
This will create speed.png which starts at 12:00 and ends at 13:00. There is a definition of a variable called myspeed, using the data from RRA ``speed'' out of database ``test.rrd''. The line drawn is 2 pixels high and represents the variable myspeed. The color is red (specified by its rgb-representation, see below).
You'll notice that the start of the graph is not at 12:00 but at 12:05. This is because we have insufficient data to tell the average before that time. This will only happen when you miss some samples, this will not happen a lot, hopefully.
If this has worked: congratulations! If not, check what went wrong.
The colors are built up from red, green and blue. For each of the components, you specify how much to use in hexadecimal where 00 means not included and FF means fully included. The ``color'' white is a mixture of red, green and blue: FFFFFF The ``color'' black is all colors off: 000000
red #FF0000 green #00FF00 blue #0000FF magenta #FF00FF (mixed red with blue) gray #555555 (one third of all components)
Additionally you can (with a recent RRDtool) add an alpha channel (transparency). The default will be ``FF'' which means non-transparent.
The PNG you just created can be displayed using your favorite image viewer. Web browsers will display the PNG via the URL ``file:///the/path/to/speed.png''
The vertical axis displays the range we entered. We provided kilometers and when divided by 300 seconds, we get very small numbers. To be exact, the first value was 12 (12357-12345) and divided by 300 this makes 0.04, which is displayed by RRDtool as ``40 m'' meaning ``40/1000''. The ``m'' (milli) has nothing to do with meters (also m), kilometers or millimeters! RRDtool doesn't know about the physical units of our data, it just works with dimensionless numbers.
If we had measured our distances in meters, this would have been (12357000-12345000)/300 = 12000/300 = 40.
As most people have a better feel for numbers in this range, we'll correct that. We could recreate our database and store the correct data, but there is a better way: we do some calculations while creating the png file!
rrdtool graph speed2.png \ --start 920804400 --end 920808000 \ --vertical-label m/s \ DEF:myspeed=test.rrd:speed:AVERAGE \ CDEF:realspeed=myspeed,1000,\* \ LINE2:realspeed#FF0000
Note: I need to escape the multiplication operator * with a backslash. If I don't, the operating system may interpret it and use it for file name expansion. You could also place the line within quotation marks like so:
It boils down to: it is RRDtool which should see *, not your shell. And it is your shell interpreting \, not RRDtool. You may need to adjust examples accordingly if you happen to use an operating system or shell which behaves differently.
After viewing this PNG, you notice the ``m'' (milli) has disappeared. This it what the correct result would be. Also, a label has been added to the image. Apart from the things mentioned above, the PNG should look the same.
The calculations are specified in the CDEF part above and are in Reverse Polish Notation (``RPN''). What we requested RRDtool to do is: ``take the data source myspeed and the number 1000; multiply those''. Don't bother with RPN yet, it will be explained later on in more detail. Also, you may want to read my tutorial on CDEFs and Steve Rader's tutorial on RPN. But first finish this tutorial.
Hang on! If we can multiply values with 1000, it should also be possible to display kilometers per hour from the same data!
To change a value that is measured in meters per second:
Calculate meters per hour: value * 3600 Calculate kilometers per hour: value / 1000 Together this makes: value * (3600/1000) or value * 3.6
In our example database we made a mistake and we need to compensate for this by multiplying with 1000. Applying that correction:
value * 3.6 * 1000 == value * 3600
Now let's create this PNG, and add some more magic ...
rrdtool graph speed3.png \ --start 920804400 --end 920808000 \ --vertical-label km/h \ DEF:myspeed=test.rrd:speed:AVERAGE \ "CDEF:kmh=myspeed,3600,*" \ CDEF:fast=kmh,100,GT,kmh,0,IF \ CDEF:good=kmh,100,GT,0,kmh,IF \ HRULE:100#0000FF:"Maximum allowed" \ AREA:good#00FF00:"Good speed" \ AREA:fast#FF0000:"Too fast"
Note: here we use another means to escape the * operator by enclosing the whole string in double quotes.
This graph looks much better. Speed is shown in km/h and there is even an extra line with the maximum allowed speed (on the road I travel on). I also changed the colors used to display speed and changed it from a line into an area.
The calculations are more complex now. For speed measurements within the speed limit they are:
Check if kmh is greater than 100 ( kmh,100 ) GT If so, return 0, else kmh ((( kmh,100 ) GT ), 0, kmh) IF
For values above the speed limit:
Check if kmh is greater than 100 ( kmh,100 ) GT If so, return kmh, else return 0 ((( kmh,100) GT ), kmh, 0) IF
rrdtool graph speed4.png \ --start 920804400 --end 920808000 \ --vertical-label km/h \ DEF:myspeed=test.rrd:speed:AVERAGE \ CDEF:nonans=myspeed,UN,0,myspeed,IF \ CDEF:kmh=nonans,3600,* \ CDEF:fast=kmh,100,GT,100,0,IF \ CDEF:over=kmh,100,GT,kmh,100,-,0,IF \ CDEF:good=kmh,100,GT,0,kmh,IF \ HRULE:100#0000FF:"Maximum allowed" \ AREA:good#00FF00:"Good speed" \ AREA:fast#550000:"Too fast" \ STACK:over#FF0000:"Over speed"
Remember the note in the beginning? I had to remove unknown data from this example. The 'nonans' CDEF is new, and the 6th line (which used to be the 5th line) used to read 'CDEF:kmh=myspeed,3600,*'
Let's create a quick and dirty HTML page to view the three PNGs:
<HTML><HEAD><TITLE>Speed</TITLE></HEAD><BODY> <IMG src="speed2.png" alt="Speed in meters per second"> <BR> <IMG src="speed3.png" alt="Speed in kilometers per hour"> <BR> <IMG src="speed4.png" alt="Traveled too fast?"> </BODY></HTML>
Name the file ``speed.html'' or similar, and look at it in your web browser.
Now, all you have to do is measure the values regularly and update the database. When you want to view the data, recreate the PNGs and make sure to refresh them in your browser. (Note: just clicking reload may not be enough, especially when proxies are involved. Try shift-reload or ctrl-F5).
perl -e 'print time, "\n" '
How to run a program on regular intervals is OS specific. But here is an example in pseudo code:
- Get the value and put it in variable "$speed" - rrdtool update speed.rrd N:$speed
(do not try this with our test database, we'll use it in further examples)
This is all. Run the above script every five minutes. When you need to know what the graphs look like, run the examples above. You could put them in a script as well. After running that script, view the page index.html we created above.
Many people interested in RRDtool will use the counter that keeps track of octets (bytes) transfered by a network device. So let's do just that next. We will start with a description of how to collect data.
Some people will make a remark that there are tools which can do this data collection for you. They are right! However, I feel it is important that you understand they are not necessary. When you have to determine why things went wrong you need to know how they work.
One tool used in the example has been talked about very briefly in the beginning of this document, it is called SNMP. It is a way of talking to networked equipment. The tool I use below is called ``snmpget'' and this is how it works:
snmpget device password OID
snmpget -v[version] -c[password] device OID
For device you substitute the name, or the IP address, of your device. For password you use the ``community read string'' as it is called in the SNMP world. For some devices the default of ``public'' might work, however this can be disabled, altered or protected for privacy and security reasons. Read the documentation that comes with your device or program.
Then there is this parameter, called OID, which means ``object identifier''.
When you start to learn about SNMP it looks very confusing. It isn't all that difficult when you look at the Management Information Base (``MIB''). It is an upside-down tree that describes data, with a single node as the root and from there a number of branches. These branches end up in another node, they branch out, etc. All the branches have a name and they form the path that we follow all the way down. The branches that we follow are named: iso, org, dod, internet, mgmt and mib-2. These names can also be written down as numbers and are 1 3 6 1 2 1.
There is a lot of confusion about the leading dot that some programs use. There is *no* leading dot in an OID. However, some programs can use the above part of OIDs as a default. To indicate the difference between abbreviated OIDs and full OIDs they need a leading dot when you specify the complete OID. Often those programs will leave out the default portion when returning the data to you. To make things worse, they have several default prefixes ...
Ok, lets continue to the start of our OID: we had 220.127.116.11.2.1 From there, we are especially interested in the branch ``interfaces'' which has number 2 (e.g., 18.104.22.168.2.1.2 or 22.214.171.124.2.1.interfaces).
First, we have to get some SNMP program. First look if there is a pre-compiled package available for your OS. This is the preferred way. If not, you will have to get the sources yourself and compile those. The Internet is full of sources, programs etc. Find information using a search engine or whatever you prefer.
Assume you got the program. First try to collect some data that is available on most systems. Remember: there is a short name for the part of the tree that interests us most in the world we live in!
I will give an example which can be used on Fedora Core 3. If it doesn't work for you, work your way through the manual of snmp and adapt the example to make it work.
snmpget -v2c -c public myrouter system.sysDescr.0
The device should answer with a description of itself, perhaps an empty one. Until you got a valid answer from a device, perhaps using a different ``password'', or a different device, there is no point in continuing.
snmpget -v2c -c public myrouter interfaces.ifNumber.0
Hopefully you get a number as a result, the number of interfaces. If so, you can carry on and try a different program called ``snmpwalk''.
snmpwalk -v2c -c public myrouter interfaces.ifTable.ifEntry.ifDescr
If it returns with a list of interfaces, you're almost there.
Here's an example:
[user@host /home/alex]$ snmpwalk -v2c -c public cisco 126.96.36.199
interfaces.ifTable.ifEntry.ifDescr.1 = "BRI0: B-Channel 1" interfaces.ifTable.ifEntry.ifDescr.2 = "BRI0: B-Channel 2" interfaces.ifTable.ifEntry.ifDescr.3 = "BRI0" Hex: 42 52 49 30 interfaces.ifTable.ifEntry.ifDescr.4 = "Ethernet0" interfaces.ifTable.ifEntry.ifDescr.5 = "Loopback0"
On this cisco equipment, I would like to monitor the ``Ethernet0'' interface and from the above output I see that it is number four. I try:
[user@host /home/alex]$ snmpget -v2c -c public cisco 188.8.131.52.4 184.108.40.206.4 interfaces.ifTable.ifEntry.ifInOctets.4 = 2290729126 interfaces.ifTable.ifEntry.ifOutOctets.4 = 1256486519
So now I have two OIDs to monitor and they are (in full, this time):
both with an interface number of 4.
Don't get fooled, this wasn't my first try. It took some time for me too to understand what all these numbers mean. It does help a lot when they get translated into descriptive text... At least, when people are talking about MIBs and OIDs you know what it's all about. Do not forget the interface number (0 if it is not interface dependent) and try snmpwalk if you don't get an answer from snmpget.
If you understand the above section and get numbers from your device, continue on with this tutorial. If not, then go back and re-read this part.
1 sample "averaged" stays 1 period of 5 minutes 6 samples averaged become one average on 30 minutes 24 samples averaged become one average on 2 hours 288 samples averaged become one average on 1 day
Lets try to be compatible with MRTG which stores about the following amount of data:
600 5-minute samples: 2 days and 2 hours 600 30-minute samples: 12.5 days 600 2-hour samples: 50 days 732 1-day samples: 732 days
These ranges are appended, so the total amount of data stored in the database is approximately 797 days. RRDtool stores the data differently, it doesn't start the ``weekly'' archive where the ``daily'' archive stopped. For both archives the most recent data will be near ``now'' and therefore we will need to keep more data than MRTG does!
We will need:
600 samples of 5 minutes (2 days and 2 hours) 700 samples of 30 minutes (2 days and 2 hours, plus 12.5 days) 775 samples of 2 hours (above + 50 days) 797 samples of 1 day (above + 732 days, rounded up to 797) rrdtool create myrouter.rrd \ DS:input:COUNTER:600:U:U \ DS:output:COUNTER:600:U:U \ RRA:AVERAGE:0.5:1:600 \ RRA:AVERAGE:0.5:6:700 \ RRA:AVERAGE:0.5:24:775 \ RRA:AVERAGE:0.5:288:797 \ RRA:MAX:0.5:1:600 \ RRA:MAX:0.5:6:700 \ RRA:MAX:0.5:24:775 \ RRA:MAX:0.5:288:797
Next thing to do is to collect data and store it. Here is an example. It is written partially in pseudo code, you will have to find out what to do exactly on your OS to make it work.
while not the end of the universe do get result of snmpget router community 220.127.116.11.4 into variable $in get result of snmpget router community 18.104.22.168.4 into variable $out rrdtool update myrouter.rrd N:$in:$out wait for 5 minutes done
Then, after collecting data for a day, try to create an image using:
rrdtool graph myrouter-day.png --start -86400 \ DEF:inoctets=myrouter.rrd:input:AVERAGE \ DEF:outoctets=myrouter.rrd:output:AVERAGE \ AREA:inoctets#00FF00:"In traffic" \ LINE1:outoctets#0000FF:"Out traffic"
This should produce a picture with one day worth of traffic. One day is 24 hours of 60 minutes of 60 seconds: 24*60*60=86400, we start at now minus 86400 seconds. We define (with DEFs) inoctets and outoctets as the average values from the database myrouter.rrd and draw an area for the ``in'' traffic and a line for the ``out'' traffic.
View the image and keep logging data for a few more days. If you like, you could try the examples from the test database and see if you can get various options and calculations to work.
Suggestion: Display in bytes per second and in bits per second. Make the Ethernet graphics go red if they are over four megabits per second.
Recall all the stuff about the speed of the car. Suppose we drove at 144 km/h during 5 minutes and then were stopped by the police for 25 minutes. At the end of the lecture we would take our laptop and create and view the image taken from the database. If we look at the second RRA we did create, we would have the average from 6 samples. The samples measured would be 144+0+0+0+0+0=144, divided by 30 minutes, corrected for the error by 1000, translated into km/h, with a result of 24 km/h. I would still get a ticket but not for speeding anymore :)
Obviously, in this case we shouldn't look at the averages. In some cases they are handy. If you want to know how many km you had traveled, the averaged picture would be the right one to look at. On the other hand, for the speed that we traveled at, the maximum numbers seen is much more interesting. Later we will see more types.
It is the same for data. If you want to know the amount, look at the averages. If you want to know the rate, look at the maximum. Over time, they will grow apart more and more. In the last database we have created, there are two archives that keep data per day. The archive that keeps averages will show low numbers, the archive that shows maxima will have higher numbers.
For my car this would translate in averages per day of 96/24=4 km/h (as I travel about 94 kilometers on a day) during working days, and maxima of 120 km/h (my top speed that I reach every day).
Big difference. Do not look at the second graph to estimate the distances that I travel and do not look at the first graph to estimate my speed. This will work if the samples are close together, as they are in five minutes, but not if you average.
On some days, I go for a long ride. If I go across Europe and travel for 12 hours, the first graph will rise to about 60 km/h. The second one will show 180 km/h. This means that I traveled a distance of 60 km/h times 24 h = 1440 km. I did this with a higher speed and a maximum around 180 km/h. However, it probably doesn't mean that I traveled for 8 hours at a constant speed of 180 km/h!
This is a real example: go with the flow through Germany (fast!) and stop a few times for gas and coffee. Drive slowly through Austria and the Netherlands. Be careful in the mountains and villages. If you would look at the graphs created from the five-minute averages you would get a totally different picture. You would see the same values on the average and maximum graphs (provided I measured every 300 seconds). You would be able to see when I stopped, when I was in top gear, when I drove over fast highways etc. The granularity of the data is much higher, so you can see more. However, this takes 12 samples per hour, or 288 values per day, so it would be a lot of data over a longer period of time. Therefore we average it, eventually to one value per day. From this one value, we cannot see much detail, of course.
Make sure you understand the last few paragraphs. There is no value in only a line and a few axis, you need to know what they mean and interpret the data in an appropriate way. This is true for all data.
The biggest mistake you can make is to use the collected data for something that it is not suitable for. You would be better off if you didn't have the graph at all.
RRDtool can do more than what we have learned up to now. Before you continue with the rest of this doc, I recommend that you reread from the start and try some modifications on the examples. Make sure you fully understand everything. It will be worth the effort and helps you not only with the rest of this tutorial, but also in your day to day monitoring long after you read this introduction.
You know that in order to view a counter over time, you have to take two numbers and divide the difference of them between the time lapsed. This makes sense for the examples I gave you but there are other possibilities. For instance, I'm able to retrieve the temperature from my router in three places namely the inlet, the so called hot-spot and the exhaust. These values are not counters. If I take the difference of the two samples and divide that by 300 seconds I would be asking for the temperature change per second. Hopefully this is zero! If not, the computer room is probably on fire :)
So, what can we do? We can tell RRDtool to store the values we measure directly as they are (this is not entirely true but close enough). The graphs we make will look much better, they will show a rather constant value. I know when the router is busy (it works -> it uses more electricity -> it generates more heat -> the temperature rises). I know when the doors are left open (the room is air conditioned) -> the warm air from the rest of the building flows into the computer room -> the inlet temperature rises). Etc. The data type we use when creating the database before was counter, we now have a different data type and thus a different name for it. It is called GAUGE. There are more such data types:
- COUNTER we already know this one - GAUGE we just learned this one - DERIVE - ABSOLUTE
The two additional types are DERIVE and ABSOLUTE. Absolute can be used like counter with one difference: RRDtool assumes the counter is reset when it's read. That is: its delta is known without calculation by RRDtool whereas RRDtool needs to calculate it for the counter type. Example: our first example (12345, 12357, 12363, 12363) would read: unknown, 12, 6, 0. The rest of the calculations stay the same. The other one, derive, is like counter. Unlike counter, it can also decrease so it can have a negative delta. Again, the rest of the calculations stay the same.
Let's try them all:
rrdtool create all.rrd --start 978300900 \ DS:a:COUNTER:600:U:U \ DS:b:GAUGE:600:U:U \ DS:c:DERIVE:600:U:U \ DS:d:ABSOLUTE:600:U:U \ RRA:AVERAGE:0.5:1:10 rrdtool update all.rrd \ 978301200:300:1:600:300 \ 978301500:600:3:1200:600 \ 978301800:900:5:1800:900 \ 978302100:1200:3:2400:1200 \ 978302400:1500:1:2400:1500 \ 978302700:1800:2:1800:1800 \ 978303000:2100:4:0:2100 \ 978303300:2400:6:600:2400 \ 978303600:2700:4:600:2700 \ 978303900:3000:2:1200:3000 rrdtool graph all1.png -s 978300600 -e 978304200 -h 400 \ DEF:linea=all.rrd:a:AVERAGE LINE3:linea#FF0000:"Line A" \ DEF:lineb=all.rrd:b:AVERAGE LINE3:lineb#00FF00:"Line B" \ DEF:linec=all.rrd:c:AVERAGE LINE3:linec#0000FF:"Line C" \ DEF:lined=all.rrd:d:AVERAGE LINE3:lined#000000:"Line D"
This translates in the following values, starting at 23:10 and ending at 00:10 the next day (where ``u'' means unknown/unplotted):
- Line A: u u 1 1 1 1 1 1 1 1 1 u - Line B: u 1 3 5 3 1 2 4 6 4 2 u - Line C: u u 2 2 2 0 -2 -6 2 0 2 u - Line D: u 1 2 3 4 5 6 7 8 9 10 u
If your PNG shows all this, you know you have entered the data correctly, the RRDtool executable is working properly, your viewer doesn't fool you, and you successfully entered the year 2000 :)
You could try the same example four times, each time with only one of the lines.
Let's go over the data again:
Why is it that the first point is unknown? We do know what we put into the database, right? True, But we didn't have a value to calculate the delta from, so we don't know where we started. It would be wrong to assume we started at zero so we don't!
Delta = 7 - 999987 = -999980 (instead of 1000007-999987=20) Real delta = -999980 + 999999 + 1 = 20
At the time of writing this document, RRDtool knows of counters that are either 32 bits or 64 bits of size. These counters can handle the following different values:
- 32 bits: 0 .. 4294967295 - 64 bits: 0 .. 18446744073709551615
If these numbers look strange to you, you can view them in their hexadecimal form:
- 32 bits: 0 .. FFFFFFFF - 64 bits: 0 .. FFFFFFFFFFFFFFFF
RRDtool handles both counters the same. If an overflow occurs and the delta would be negative, RRDtool first adds the maximum of a small counter + 1 to the delta. If the delta is still negative, it had to be the large counter that wrapped. Add the maximum possible value of the large counter + 1 and subtract the erroneously added small value.
There is a risk in this: suppose the large counter wrapped while adding a huge delta, it could happen, theoretically, that adding the smaller value would make the delta positive. In this unlikely case the results would not be correct. The increase should be nearly as high as the maximum counter value for that to happen, so chances are you would have several other problems as well and this particular problem would not even be worth thinking about. Even though, I did include an example, so you can judge for yourself.
The next section gives you some numerical examples for counter-wraps. Try to do the calculations yourself or just believe me if your calculator can't handle the numbers :)
- 32 bits: (4294967295 + 1) = 4294967296 - 64 bits: (18446744073709551615 + 1) - correction1 = 18446744069414584320 Before: 4294967200 Increase: 100 Should become: 4294967300 But really is: 4 Delta: -4294967196 Correction1: -4294967196 + 4294967296 = 100 Before: 18446744073709551000 Increase: 800 Should become: 18446744073709551800 But really is: 184 Delta: -18446744073709550816 Correction1: -18446744073709550816 + 4294967296 = -18446744069414583520 Correction2: -18446744069414583520 + 18446744069414584320 = 800 Before: 18446744073709551615 ( maximum value ) Increase: 18446744069414584320 ( absurd increase, minimum for Should become: 36893488143124135935 this example to work ) But really is: 18446744069414584319 Delta: -4294967296 Correction1: -4294967296 + 4294967296 = 0 (not negative -> no correction2) Before: 18446744073709551615 ( maximum value ) Increase: 18446744069414584319 ( one less increase ) Should become: 36893488143124135934 But really is: 18446744069414584318 Delta: -4294967297 Correction1: -4294967297 + 4294967296 = -1 Correction2: -1 + 18446744069414584320 = 18446744069414584319
As you can see from the last two examples, you need strange numbers for RRDtool to fail (provided it's bug free of course), so this should not happen. However, SNMP or whatever method you choose to collect the data, might also report wrong numbers occasionally. We can't prevent all errors, but there are some things we can do. The RRDtool ``create'' command takes two special parameters for this. They define the minimum and maximum allowed values. Until now, we used ``U'', meaning ``unknown''. If you provide values for one or both of them and if RRDtool receives data points that are outside these limits, it will ignore those values. For a thermometer in degrees Celsius, the absolute minimum is just under -273. For my router, I can assume this minimum is much higher so I would set it to 10, where as the maximum temperature I would set to 80. Any higher and the device would be out of order.
For the speed of my car, I would never expect negative numbers and also I would not expect a speed higher than 230. Anything else, and there must have been an error. Remember: the opposite is not true, if the numbers pass this check, it doesn't mean that they are correct. Always judge the graph with a healthy dose of suspicion if it seems weird to you.
Suppose a counter increases by exactly one for every second. You want to measure it in 300 seconds intervals. You should retrieve values that are exactly 300 apart. However, due to various circumstances you are a few seconds late and the interval is 303. The delta will also be 303 in that case. Obviously, RRDtool should not put 303 in the database and make you believe that the counter increased by 303 in 300 seconds. This is where RRDtool interpolates: it alters the 303 value as if it would have been stored earlier and it will be 300 in 300 seconds. Next time you are at exactly the right time. This means that the current interval is 297 seconds and also the counter increased by 297. Again, RRDtool interpolates and stores 300 as it should be.
in the RRD in reality time+000: 0 delta="U" time+000: 0 delta="U" time+300: 300 delta=300 time+300: 300 delta=300 time+600: 600 delta=300 time+603: 603 delta=303 time+900: 900 delta=300 time+900: 900 delta=297
Let's create two identical databases. I've chosen the time range 920805000 to 920805900 as this goes very well with the example numbers.
rrdtool create seconds1.rrd \ --start 920804700 \ DS:seconds:COUNTER:600:U:U \ RRA:AVERAGE:0.5:1:24
Make a copy
for Unix: cp seconds1.rrd seconds2.rrd for Dos: copy seconds1.rrd seconds2.rrd for vms: how would I know :)
Put in some data
rrdtool update seconds1.rrd \ 920805000:000 920805300:300 920805600:600 920805900:900 rrdtool update seconds2.rrd \ 920805000:000 920805300:300 920805603:603 920805900:900
rrdtool graph seconds1.png \ --start 920804700 --end 920806200 \ --height 200 \ --upper-limit 1.05 --lower-limit 0.95 --rigid \ DEF:seconds=seconds1.rrd:seconds:AVERAGE \ CDEF:unknown=seconds,UN \ LINE2:seconds#0000FF \ AREA:unknown#FF0000 rrdtool graph seconds2.png \ --start 920804700 --end 920806200 \ --height 200 \ --upper-limit 1.05 --lower-limit 0.95 --rigid \ DEF:seconds=seconds2.rrd:seconds:AVERAGE \ CDEF:unknown=seconds,UN \ LINE2:seconds#0000FF \ AREA:unknown#FF0000
View both images together (add them to your index.html file) and compare. Both graphs should show the same, despite the input being different.
Alex van den Bogaerdt <email@example.com>