From: oetiker Date: Sun, 24 Apr 2005 17:23:57 +0000 (+0000) Subject: big patch-up for the 1.2. Documents Revisited by Fritz Zaucker X-Git-Url: https://git.verplant.org/?a=commitdiff_plain;h=e4acaa789385e98fe979cd48cb0fb1ddc8570b91;p=rrdtool.git big patch-up for the 1.2. Documents Revisited by Fritz Zaucker git-svn-id: svn://svn.oetiker.ch/rrdtool/branches/1.2/program@422 a5681a0c-68f1-0310-ab6d-d61299d08faa --- diff --git a/doc/rrddump.pod b/doc/rrddump.pod index 7392518..f0b446a 100644 --- a/doc/rrddump.pod +++ b/doc/rrddump.pod @@ -1,6 +1,6 @@ =head1 NAME -rrddump - Dump the contents of an RRD to XML format +rrddump - dump the contents of an RRD to XML format =head1 SYNOPSIS @@ -10,8 +10,8 @@ B B I E I The B function prints the contents of an B in human readable (?) XML format. This format can be read by rrdrestore. -Together they allow you to transfer your files from one architecture -to another as well as manipulating the contents of an B file in a +Together they allow you to transfer your files from one computer architecture +to another as well to manipulate the contents of an B file in a somewhat more convenient manner. @@ -37,11 +37,11 @@ to export the data to XML format. =item 2. -Transfer the XML dump to the target system +Transfer the XML dump to the target system. =item 3. -Run B B to create a new RRD from the XML dump. See +Run B B to create a new RRD from the XML dump. See B for details. =back diff --git a/doc/rrdfirst.pod b/doc/rrdfirst.pod index 63ef79e..e5d91de 100644 --- a/doc/rrdfirst.pod +++ b/doc/rrdfirst.pod @@ -8,8 +8,8 @@ B B I [I<--rraindex number>] =head1 DESCRIPTION -The B function returns the UNIX timestamp of the first data sample for -the specified RRA of the RRD file. +The B function returns the UNIX timestamp of the first data +sample entered into the specified RRA of the RRD file. =over 8 @@ -19,7 +19,7 @@ The name of the B that contains the data. =item I<--rraindex number> -The index number of the B that is to be examined. If not specified, the +The index number of the B that is to be examined. If not specified, the index defaults to zero. B index numbers can be determined through B. diff --git a/doc/rrdinfo.pod b/doc/rrdinfo.pod index 08f8633..24676ef 100644 --- a/doc/rrdinfo.pod +++ b/doc/rrdinfo.pod @@ -1,6 +1,6 @@ =head1 NAME -rrdinfo - Extract header information from an RRD +rrdinfo - extract header information from an RRD =head1 SYNOPSIS @@ -20,7 +20,7 @@ This is the output generated by running B on a simple RRD which contains two data sources and one RRA. Note that the number after the I keyword is in seconds since 1970. The string B stands for I<*UNKNOWN*> data. In the example it means that this RRD -has neither minimum not maximum values defined for either of its +has neither minimum nor maximum values defined for either of its data sources. filename = "random.rrd" @@ -52,7 +52,7 @@ data sources. =item I -The name of the B you want to dump. +The name of the B you want to examine. =back diff --git a/doc/rrdlast.pod b/doc/rrdlast.pod index d709c5a..a3eb7fe 100644 --- a/doc/rrdlast.pod +++ b/doc/rrdlast.pod @@ -8,8 +8,8 @@ B B I =head1 DESCRIPTION -The B function returns the UNIX timestamp when the RRD was last -updated. +The B function returns the UNIX timestamp of the most recent +update of the RRD. =over 8 diff --git a/doc/rrdresize.pod b/doc/rrdresize.pod index 6342cd8..4165a18 100644 --- a/doc/rrdresize.pod +++ b/doc/rrdresize.pod @@ -1,6 +1,6 @@ =head1 NAME -rrdresize - Alter the size of an RRA +rrdresize - alters the size of an RRA and creates a new .rrd file =head1 SYNOPSIS @@ -44,7 +44,7 @@ B in the current directory. B. It is possible to abuse this tool and get strange results -by first removing some rows and then reinsert the same amount (effectively +by first removing some rows and then reinserting the same amount (effectively clearing them to be Unknown). You may thus end up with unknown data in one RRA while at the same timestamp this data is available in another RRA. diff --git a/doc/rrdrestore.pod b/doc/rrdrestore.pod index 7f78d27..811548d 100644 --- a/doc/rrdrestore.pod +++ b/doc/rrdrestore.pod @@ -10,13 +10,13 @@ S<[B<--range-check>|B<-r>]> =head1 DESCRIPTION The B function reads the XML representation of an RRD and converts -it into the native B format. +it to the native B format. =over 8 =item I -The name of the B you want to restore. +The name of the B file you want to restore. =item I @@ -25,7 +25,7 @@ The name of the B to restore. =item B<--range-check>|B<-r> Make sure the values in the RRAs do not exceed the limits defined for -the different data sources. +the various data sources. =item B<--force-overwrite>|B<-f> diff --git a/doc/rrdthreads.pod b/doc/rrdthreads.pod index a1f6e74..7769018 100644 --- a/doc/rrdthreads.pod +++ b/doc/rrdthreads.pod @@ -9,25 +9,25 @@ precautions, as the RRD library in its original form was not thread-safe at all. This document describes requirements and pitfalls on the way to use the multi-threaded version of librrd in your own programs. It also gives hints for future RRD development to keep the -library thread-safe.. +library thread-safe. Currently only some RRD operations are implemented in a thread-safe -way. They all end in the usual "C<_r>" prefix. +way. They all end in the usual "C<_r>" suffix. =head1 DESCRIPTION -In order to use the librrd in multi-threaded programs you must: +In order to use librrd in multi-threaded programs you must: =over =item * -Link with F instead of with F (use C<-lrrd_th> when +Link with F instead of F (use C<-lrrd_th> when linking) =item * -Use the "C<_r>" functions instead or the normal API-functions +Use the "C<_r>" functions instead of the normal API-functions =item * @@ -37,7 +37,7 @@ specifications is terribly non-thread-safe. =item * Never use non *C<_r> functions unless it is explicitly documented that -the function is tread-safe +the function is tread-safe. =item * @@ -64,7 +64,7 @@ Some precautions must be followed when developing RRD from now on: Only use thread-safe functions in library code. Many often used libc functions aren't thread-safe. Take care in the following -situations/when using the following library functions: +situations or when using the following library functions: =over @@ -93,7 +93,7 @@ C: use C =item * -Many other (lookup documentation) +Many others (lookup documentation) =back @@ -115,8 +115,8 @@ F and F =item * -Do not use C or C in *C<_r> (directly or -indirectly) +Do not use C or C in *C<_r> (neither directly nor +indirectly). C uses global variables and behaves badly in a multi-threaded application when called concurrently. Instead provide a *_r function @@ -128,15 +128,15 @@ C as an example. Do not use the C function! -It uses lots of global vars. You may use it in functions not designed -to be thread-safe like functions wrapping the C<_r> version of some +It uses lots of global variables. You may use it in functions not designed +to be thread-safe, like in functions wrapping the C<_r> version of some operation (e.g., C, but not in C) =back =head2 CURRENTLY IMPLEMENTED THREAD SAFE FUNCTIONS -Currently there exit thread-safe variants of C, +Currently there exist thread-safe variants of C, C, C, C and C. =head1 AUTHOR diff --git a/doc/rrdtool-xport.dtd b/doc/rrdtool-xport.dtd index 4f575e3..0e68015 100644 --- a/doc/rrdtool-xport.dtd +++ b/doc/rrdtool-xport.dtd @@ -2,7 +2,7 @@ - + diff --git a/doc/rrdtutorial.pod b/doc/rrdtutorial.pod index 3c9318a..4a606c5 100644 --- a/doc/rrdtutorial.pod +++ b/doc/rrdtutorial.pod @@ -23,62 +23,69 @@ Please don't skip ahead in this document! The first part of this document explains the basics and may be boring. But if you don't understand the basics, the examples will not be as meaningful to you. -=head2 What is RRDtool ? +=head2 What is RRDtool? RRDtool refers to Round Robin Database tool. Round robin is a technique that works with a fixed amount of data, and a pointer to the current element. Think of a circle with some dots plotted -on the edge, these dots are the places where data can be stored. Draw an -arrow from the center of the circle to one of the dots, this is the pointer. +on the edge -- these dots are the places where data can be stored. Draw an +arrow from the center of the circle to one of the dots -- this is the pointer. When the current data is read or written, the pointer moves to the next -element. As we are on a circle there is no beginning nor an end, you can -go on and on. After a while, all the available places will be used and -the process automatically reuses old locations. This way, the database +element. As we are on a circle there is neither a beginning nor an end, you can +go on and on and on. After a while, all the available places will be used and +the process automatically reuses old locations. This way, the dataset will not grow in size and therefore requires no maintenance. RRDtool works with with Round Robin Databases (RRDs). It stores and retrieves data from them. -=head2 What data can be put into an RRD ? - -You name it, it will probably fit. You should be able to measure some value -at several points in time and provide this information to RRDtool. If you -can do this, RRDtool will be able to store it. The values need to be -numerical but don't have to be, as opposed to MRTG, integers. - -Many examples 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 is a way to -ask devices for the values of counters they keep. -It is the value from those counters that are kept in the RRD. - -=head2 What can I do with this tool ? - -RRDtool originated from MRTG (Multi Router Traffic Grapher). MRTG started -as a tiny little script for graphing the use of a connection -to the Internet. MRTG evolved into a tool for graphing other data sources -including temperature, speed, voltage, number of printouts and -the like. Most likely you will start to use the 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. -RRDtool 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. -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 need a sensor to -measure the data and be able to feed the numbers to RRDtool. - -=head2 What if I still have problems after reading this document ? +=head2 What data can be put into an RRD? + +XXX time series ??? XXX You name it, it will probably fit. You should +be able to measure some value at several points in time and provide +this information to RRDtool. If you can do this, RRDtool will be able +to store it. The values must be numerical but don't have to be +integers, as is the case with MRTG (the next section will give more +details on this more specialized application). + +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. + +=head2 What can I do with this tool? + +RRDtool originated from MRTG (Multi Router Traffic Grapher). MRTG +started as a tiny little script for graphing the use of a university's +connection to the Internet. MRTG was later (ab-)used as a tool for +graphing other data sources including temperature, speed, voltage, +number of printouts and the like. + +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. + +=head2 What if I still have problems after reading this document? First of all: read it again! You may have missed something. If you are unable to compile the sources and you have a fairly common OS, it will probably not be the fault of RRDtool. There may be pre-compiled versions around on the Internet. If they come from trusted sources, get one of those. + 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. @@ -87,21 +94,22 @@ 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 list! Look in the documentation that +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 Errd-users-request@list.ee.ethz.chE with a -subject of "subscribe". If you ever want to leave this list, you write +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". -=head2 How will you help me ? +=head2 How will you help me? By giving you some detailed descriptions with detailed examples. -It is assumed that following the instructions in the order presented +I assume that following the instructions in the order presented will give you enough knowledge of RRDtool to experiment for yourself. If it doesn't work the first time, don't give up. Reread the stuff that you did understand, you may have missed something. + By following the examples you get some hands-on experience and, even more important, some background information of how it works. @@ -121,13 +129,14 @@ 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 have -a maximum value, if that value is reached and an extra byte is counted, -the counter starts all over at zero. This is the same as many counters +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 ! +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 @@ -143,7 +152,7 @@ Take the current counter, subtract the previous value from it. =item 2. -Do the same with the current time and the previous time. +Do the same with the current time and the previous time (in seconds). =item 3. @@ -155,8 +164,8 @@ number of bits per second (bps). bps = (counter_now - counter_before) / (time_now - time_before) * 8 -For some people it may help to translate this to a automobile example: -Do not try this example, and if you do, don't blame me for the results. +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). @@ -169,7 +178,7 @@ I will use the following abbreviations: KM/H: kilometers per hour M/S: meters per second -You're driving a car. At 12:05 you read the counter in the dashboard +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 @@ -177,11 +186,10 @@ 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 which translates into 300 seconds. Our speed is 12000M / 300S -equals 40 M/S. +minutes or 300 seconds. Our speed is 12000M / 300S or 40 M/S. -We could also calculate the speed in KM/H: 12 times five minutes -is an hour so we have to multiply 12 KM by 12 to get 144 KM/H. +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 :) @@ -195,8 +203,8 @@ 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 OS. Make sure you do not overwrite any file on your system when +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. @@ -209,22 +217,27 @@ and skip all of the '\' characters. (So enter: C) -=head2 What has been created ? +=head2 What has been created? -We created the round robin database called test (test.rrd) -which starts at noon the day I started (7th of march, 1999) writing -this document. It holds one data source (DS) named "speed" that gets -built from a counter. This counter is read every five minutes (default) -In the same database two round robin archives (RRAs) are kept, one -averages the data every time it is read (e.g., there's nothing to average) -and keeps 24 samples (24 times 5 minutes is 2 hours). The other averages -6 values (half hour) and contains 10 of such averages (e.g., 5 hours) -The remaining options will be discussed later on. +We created the round robin database called test (test.rrd) which +starts at noon the day I started writing this document, 7th of March, +1999 (this date translates to 920804400 seconds as explained +below). Our database holds one data source (DS) named "speed" that +represents a counter. This counter is read every five minutes +(default). In the same database two round robin archives (RRAs) are +kept, one averages the data every time it is read (e.g., there's +nothing to average) and keeps 24 samples (24 times 5 minutes is 2 +hours). The other averages 6 values (half hour) and contains 10 of +such averages (e.g., 5 hours). + +=for comment + XXX The remaining options will be discussed later on. (there aren't any + XXX in the example above, Fritz) 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. This time stamp is translated into local time and -it will therefore look different for the different time zones. +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 @@ -268,7 +281,7 @@ etcetera. 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 is OS dependent. +the real maximum per line is OS dependent. We can now retrieve the data from our database using "rrdtool fetch": @@ -296,7 +309,7 @@ It should return the following output: 920809200: nan If it doesn't, something may be wrong. Perhaps your OS will print -"NaN" in a different form. It represents "Not A Number". If your OS +"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 @@ -304,12 +317,12 @@ database and try again. 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 +form of output. The examples in this document are correct for version +1.2.0 of RRDtool. -What this output represents will become clear in the rest of the tutorial. +The meaning of the above output will become clear below. -=head2 It is time to create some graphics +=head2 Time to create some graphics Try the following command: @@ -319,11 +332,13 @@ Try the following command: LINE2:myspeed#FF0000 This will create speed.png which starts at 12:00 and ends at 13:00. -There is a definition of variable myspeed, it is the data from RRA -"speed" out of database "test.rrd". The line drawn is 2 pixels high, -and comes from variable myspeed. The color is red. -You'll notice that the start of the graph is not at 12:00 but at 12:05 -and this is because we have insufficient data to tell the average before +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. @@ -354,19 +369,22 @@ viewer. Web browsers will display the PNG via the URL When looking at the image, you notice that the horizontal axis is labeled 12:10, 12:20, 12:30, 12:40 and 12:50. Sometimes a label doesn't fit (12:00 and 13:00 would be candidates) so they are skipped. -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" has -nothing to do with meters, kilometers or millimeters! RRDtool doesn't -know about all this, it just works with numbers and not with meters... -What we did wrong was that we should have measured in meters, this would -have been (12357000-12345000)/300 = 12000/300 = 40. +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, +kilometers or millimeters! RRDtool doesn't know about the physical +units of our data, it just works with dimensionless numbers. -Let's correct that. We could recreate our database and store the correct -data but there is a better way: do some calculations while creating the -png file ! +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 \ @@ -375,23 +393,33 @@ png file ! CDEF:realspeed=myspeed,1000,\* \ LINE2:realspeed#FF0000 -After viewing this PNG, you notice the "m" 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 be the same. +Note: Make sure not to forget the backslash \ in front of the +multiplication operator * above. The backslash is needed to "escape" +the * as some operating systems might interpret and expand * instead +of passing it to the rrdtool command. + +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 in the CDEF part and are in Reverse Polish Notation -("RPN"). What it says 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. +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! +=for comment +XXX strange format below: -*- ; Fritz + 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) == value * 3.6 + -*- 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: @@ -410,16 +438,21 @@ Now let's create this PNG, and add some more magic ... AREA:good#00FF00:"Good speed" \ AREA:fast#FF0000:"Too fast" -This looks much better. Speed in KM/H and even an extra line with the -maximum allowed speed (on the road I travel at). I also changed the -colors used to display speed and changed it from a line into an area. +Note: here we use another means to escape the * operator by enclosing +the whole string in double quotes. -The calculations are more complex now. For the "good" speed they are: +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 the other speed: +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 @@ -443,7 +476,7 @@ following PNG: AREA:fast#550000:"Too fast" \ STACK:over#FF0000:"Over speed" -Let's create a quick and dirty HTML page to view three PNGs: +Let's create a quick and dirty HTML page to view the three PNGs: Speed Speed in meters per second @@ -453,7 +486,7 @@ Let's create a quick and dirty HTML page to view three PNGs: Traveled too fast? -Name the file "speed.html" or similar, and view it. +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 @@ -463,47 +496,51 @@ or ctrl-F5). =head2 Updates in Reality -We've already used the "update" command: it took one or more parameters -in the form of "