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dool-paper.html
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
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<title>Dstat: pluggable real-time monitoring</title>
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@media print {
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<body class="article">
<div id="header">
<h1>Dstat: pluggable real-time monitoring</h1>
<span id="author">Dag Wieers</span><br />
<span id="email"><code><<a href="mailto:dag@wieers.com">dag@wieers.com</a>></code></span><br />
<span id="revdate">$Id$</span>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><em>This Dstat paper was originally written for LinuxConf Europe that was
held together with the Linux Kernel summit at the University in Cambridge,
UK in August 2007.</em></p></div>
</div>
</div>
<div class="sect1">
<h2 id="_introduction">Introduction</h2>
<div class="sectionbody">
<div class="paragraph"><p>Many tools exist to monitor hardware resources and software behaviour, but few
tools exist that allow you to easily monitor any conceivable counter.</p></div>
<div class="paragraph"><p>Dstat was designed with the idea that it should be simple to plug in a piece
of code that extracts one or more counters, and make it visible in a way that
visually pleases the eye and helps you extract information in real-time.</p></div>
<div class="paragraph"><p>By being able to select those counters that you want (and likely those
counters that matter to you in the job you’re doing) you make it easier to
correlate raw numbers and see a pattern that may otherwise not be visible.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_a_case_for_dstat">A case for Dstat</h2>
<div class="sectionbody">
<div class="paragraph"><p>A few years ago I was involved in a project that was testing a storage cluster
with a SAN back-end using GPFS and Samba for a broadcasting company. The
performance tests that were scheduled together with the customer took a few
weeks to measure the different behaviour under different stresses.</p></div>
<div class="paragraph"><p>During these tests there was a need to see how each of the components behaved
and to find problematic behaviour during testing. Also, because it involved 5
GPFS nodes, we needed to make sure that the load was spread evenly during the
test. If everything went well repeatedly, the results were validated and the
next batch of tests could be prepared and run.</p></div>
<div class="paragraph"><p>We started off using different tools at first, but the more counters we were
trying to capture the harder it was to post-process the information we had
collected. What’s more, we often saw only after performing the tests that the
data was not representative because the numbers didn’t add up. Sometimes it
was caused by the massive setup of clients that were autonomously stressing the
cluster. On other occasions we noticed that the network was the culprit. All in
all, we lost time because we could only validate the results by relating
numbers after the tests were complete and not during the tests.</p></div>
<div class="paragraph"><p>Complicating the matter was the fact that 5 different nodes were involved
and using the normal command line tools like vmstat, iostat or ifstat (which
only showed us a small part of what was happening) was problematic as each
needed a different terminal. Besides, not all information was interesting.</p></div>
<div class="paragraph"><p>Eventually Dstat was born, to make a dull task more enjoyable.</p></div>
<div class="paragraph"><p>After the project was finished I was able to correlate system resources with
network throughput, TCP information, Samba sessions, GPFS throughput,
accumulated block device throughput, HBA throughput, all within a single
interval on one screen for the complete cluster.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_dstat_characteristics">Dstat characteristics</h2>
<div class="sectionbody">
<div class="paragraph"><p>There are many ideas incorporated into Dstat by design, and this section
serves to list all of them. Not all of them may appeal to the task you’re
doing, but the combination may make it an appealing proposition nevertheless.</p></div>
<div class="sect2">
<h3 id="_history_of_counters">History of counters</h3>
<div class="paragraph"><p>An important characteristic in line-based tools like vmstat, iostat or
ifstat is the fact that you can compare historical collected data with
new data. This allows you to have a good feeling of how something is
evolving.</p></div>
<div class="paragraph"><p>Compare this to tools like top or nmon, where data is often being refreshed
and you loose historical information (but in return can provide you with
a lot more information at the same time).</p></div>
</div>
<div class="sect2">
<h3 id="_adding_unit_indication">Adding unit indication</h3>
<div class="paragraph"><p>It was very important that when numbers were compared, they were in the same
unit, and not eg. a different power exponent. The human mind sometimes works
in mysterious ways and more so when working with numbers for hours and hours.
Adding the unit is something very convenient and may reduce the human error
factor.</p></div>
<div class="paragraph"><p>Additionally, indicating the unit also makes sure that the columns have a
fixed width. Often when using vmstat or other tools, the columns tend to shift
depending on the width of the counter. This makes it very inconvenient to find
counters in the shifted output.</p></div>
</div>
<div class="sect2">
<h3 id="_colour_highlighting_units">Colour highlighting units</h3>
<div class="paragraph"><p>After I added colours to help improve indicating units, I noticed that the
colours also helped to show patterns. This of course is very limited,
nevertheless it instantly shows when numbers are flat or changes are taking
place.</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Important</div>
</td>
<td class="content">The colours are arbitrarily chosen. Do not make the mistake to
assume that green means good and red means bad. There is no real meaning to
the colour itself, however a change of colour does mean that a value has gone
over some pre-defined limit.</td>
</tr></table>
</div>
</div>
<div class="sect2">
<h3 id="_intermediate_updates">Intermediate updates</h3>
<div class="paragraph"><p>During tests, when you choose to see average values over a given time, it can
be useful to see how the averages evolve. Dstat, by default, displays
intermediate updates. This means that if you select to see 10 second averages,
after each second you see the accumulated average over the timespan. <strong>This
means that after 4 seconds with intermediate updates, you see an average
taken over the 4 second timeframe.</strong></p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">This means that the closer you get to the given timeframe (eg. 10 seconds)
the more likely that it nears its final average over that period.</td>
</tr></table>
</div>
</div>
<div class="sect2">
<h3 id="_adding_custom_counters">Adding custom counters</h3>
<div class="paragraph"><p>Dstat was specifically designed to enable anyone to add their own counters in a
matter of minutes. The plugin-based system takes care of displaying, colouring
and adding units to the counters. As a plugin-writer, you only have to focus
on extracting the counters from the kernel (procfs or sysfs), logfiles or
daemons.</p></div>
</div>
<div class="sect2">
<h3 id="_selecting_plugins_and_counters">Selecting plugins and counters</h3>
<div class="paragraph"><p>Being able to add custom counters is important, but selecting those counters
that you really need is even more important if you want to correlate counters
and see patterns. Less is more.</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">In fact, Dstat currently does not allow you to select just counters, it
only allows you to select plugins. However, since you can modify or fork a
plugin, you still have the ability to select just those counters you prefer.</td>
</tr></table>
</div>
</div>
<div class="sect2">
<h3 id="_exporting_to_csv">Exporting to CSV</h3>
<div class="paragraph"><p>Having information on screen is one thing, you most likely need some hard
evidence later to make your case. (Why else do all the work?)</p></div>
<div class="paragraph"><p>Dstat allows to write out all counters in the greatest detail possible to CSV.
By default it also adds the command-line used for generating the output, as
well as a date and time stamp. Since Dstat in the first place is meant for
human-readable real-time statistics, it will by default also display the
counters to screen (unless you <em>/dev/null</em> it).</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Tip</div>
</td>
<td class="content">Dstat appends to the output file so that you can add tests-results of
different tests to a single file. However, make sure that you tag each test
properly (eg. by using distinct filenames for each different test).</td>
</tr></table>
</div>
</div>
<div class="sect2">
<h3 id="_time_plugin_included">Time-plugin included</h3>
<div class="paragraph"><p>It may seem a small thing, but having exact time (and date) information for
your counters allows for a completely different usage as well. By adding
simple date and time information, Dstat can be used as a background process in
a screen to monitor the behaviour of your system during the night.</p></div>
<div class="paragraph"><p>This proves to be very valuable for example, to find offending processes during
nightly tasks or to pinpoint their behaviour to certain events that you cannot
monitor during working hours.</p></div>
<div class="paragraph"><p>It is also important when you have multiple Dstats running (eg. for nodes in a
cluster) to correlate counters between the outputs.</p></div>
</div>
<div class="sect2">
<h3 id="_terminal_capabilities">Terminal capabilities</h3>
<div class="paragraph"><p>Dstat also takes into account the width and height of your terminal window and
modifies output to fit into your terminal. This, of course, has no effect on
what ends up in the CSV output.</p></div>
<div class="paragraph"><p>Another (debatable) useful feature is that Dstat will modify the terminal
title to indicate on what system it was run and what options were used.
Especially when monitoring nodes in a cluster, this can be useful, but even in
Gnome finding your Dstat window is handy.</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Warning</div>
</td>
<td class="content">Some people however are annoyed by the fact that their distribution
does not reset the terminal title and Dstat therefor messes it up. There is no
way for Dstat to fix this.</td>
</tr></table>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_plugins_and_counters">Plugins and counters</h2>
<div class="sectionbody">
<div class="paragraph"><p>When we talk about plugins, we make a distinction between those plugins that
are included within the Dstat tool itself, and those that ship with it
externally. In essence there is no real difference, as the internal plugins
could easily have been created as an external plugin. The basic difference is
that the internal plugins have no dependencies except on procfs.</p></div>
<div class="paragraph"><p>Having the basic plugins as part of Dstat, makes sure that Dstat can be moved
as a self-contained file to other systems.</p></div>
<div class="sect2">
<h3 id="_internal_plugins">Internal plugins</h3>
<div class="paragraph"><p>The plugins that have been selected to be part of the Dstat tool itself, and
therefor have no dependencies other than procfs, are:</p></div>
<div class="ulist"><ul>
<li>
<p>
aio: asynchronous I/O counters
</p>
</li>
<li>
<p>
cpu, cpu24: CPU counters (<code>-c</code> and <code>-C</code>)
</p>
</li>
<li>
<p>
disk, disk24, disk24old: disk counters (<code>-d</code> and <code>-D</code>)
</p>
</li>
<li>
<p>
epoch: seconds since Epoch (<code>-T</code>)
</p>
</li>
<li>
<p>
fs: file system counters
</p>
</li>
<li>
<p>
int, int24: interrupts per IRQ (<code>-i</code> and <code>-I</code>)
</p>
</li>
<li>
<p>
io: I/O requests completed (<code>-r</code>)
</p>
</li>
<li>
<p>
ipc: IPC counters
</p>
</li>
<li>
<p>
load: load counters (<code>-l</code>)
</p>
</li>
<li>
<p>
lock: locking counters
</p>
</li>