For a more comprehensive list of monitoring tools and their features, check out this Wikipedia page.
As the question states, what are the most commonly used tools used for this task and what are their strengths and weaknesses?
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I've used Nagios in the past with success. It's very extensible (over 200 add-ons), relatively easy to use and lots of reports. A negative would be the initial setup.
We're using that cacti/RRDTool solution to monitor Unix and Windows systems. We get a lot of useful metrics including load, CPU/RAM usage, HD space, users logged in, network traffic, running processes, and so on.
You will find more information on cacti on the What is Cacti? page.
Personally, I love Munin which is very easy to install and to write plugins for as it has a very straightforward architecture. There are quite many plugins already around for all the purposes you could imagine, so you probably won't even have to write plugins in the first place.
It also provides beautiful graphs and the option to configure (very basic) alerts.
Zabbix. It's open-source, and reasonably simple to setup and customise. We have a lot of custom monitoring scripts that feed into the zabbix server, but it takes care of centralising that data, displaying it appropriately, notifications (email, IM, SMS, twitter, etc), and so forth.
I have been doing roll outs of Spiceworks at our company and we are finding it to be a great tool not just for monitoring servers but everything else on the network.
It does things like automatic inventory and custom monitoring to send you emails when there is a problem (EG: Printer is down to 10% of ink or hard drive of this server has 20%).
Its downside would probably be is density of information per computer, don't get it wrong it has A LOT of data per machine but for things like servers where you might want a lot of stats you might need to use another tool.
EDIT: oh did i mention its business model is based around it being free forever.
Smokeping not only checks the availability of various servers and services but also keeps track of their latency while providing easy to use, nice looking, and quick to display graphs.
Wide range of latency measurement plugins is available out of the box. If you know some Perl, it is easy to create your own ones for any exotic needs.
Large installations will benefit from Master/Slave System for distributed measurement.
Highly configurable alerting system will help you notice issues before they start affecting users or evolve into major outage.
Smokeping is free and OpenSource Software written in Perl by Tobi Oetiker, the creator of MRTG and RRDtool
OpenNMS is used where I work to monitor more than a thousand Linux machines. We monitor the hardware of each machine and the applications running on them.
Zenoss Core is of some use, We are using it (for about a year) for lightweight monitoring of servers, net switches and UPSs.
Zenoss Core is an award-winning open source IT monitoring product that effectively manages the configuration, health and performance of networks, servers and applications through a single, integrated software package.
Nagios is great since it's free and there is plenty of plugin's for it. However the UI and config is very difficult.
It's exact opposite in pro's/con's which is also great is Microsoft System Centre Operations Manager (SCOM) which is not free, has less plugin's but setup and config are brilliant and easy.
I must admit if I was in a primarily Microsoft company, had very high reliance requirements (i.e. can't afford for monitoring to break) or had to think about getting developers to work with it then SCOM would be my recommendation over Nagios.
For monitoring statistics (memory usage, load, mysql activity, apache activity, etc.) I use Munin. Out of the box it already tracks a lot of things and plots graphs for different time intervals (last 24 hours, last 7 days, last month, last year). Through plugins even more things can be monitored. It's output are HTML pages with pretty graphs.
Munin has a master/node architecture: nodes gather statistics on a server and the master stores the data and produces HTML and graphs.
I use Monit to keep track of running processes and to restart or alert me when certain configureable conditions arise (high cpu load, high memory usage, no HTTP response, etc.) Monit can also monitor more general things about a server, such as cpu load, memory usage, harddisk status or disk usage.
Monit needs to be configured for every service or hardware you want to monitor and how to respond when something goes wrong. The most used options are to do nothing, send an alert email or restart the service.
Monit is great when it works, but sometimes it fails to start, stop or restart a service and there is not a lot of diagnostic information available to tell you what went wrong. This means you don't know if the problem was with your service or with the Monit configuration, which runs with a cron-like minimal environment.
Both tools are available by default on most Linux distributions.
I'm part of a operational monitoring upgrade project. We've had various vendors come onsite to present a few big dollar systems and mixed in some cheaper alternatives to compare.
One of which is Hyperic, which is also available as a free open source solution. I was impressed with its delivered capabilities and extensibility for custom agents.
I use Pingdom for monitoring my server. It sends me an SMS message when the server is unreachable.
It's important for us to have very low overhead on each node so that as many resources as possible are available for computation. Ganglia gives us a good overview of the cluster and allows us to drill down to individual nodes if needed. Besides know what's going on right now, we can get a pretty good look at what's happened over the last hour, day, week, month, and year. The graphs of various statistics are basic and functional.
It all depends what you mean by "monitor"!
A new entrant on the scene to check out for competing with Cacti and the RRDTool based solutions is Graphite (http://graphite.wikidot.com/)
RRDTool is replaced with a backing store called Whisper. The docs give a pretty good overview of why it differs and I really like the CLI for ad hoc graphing when investigating something.
We use (and like) WhatsUp from Ipswitch for our relatively small Windows network. It is easy to setup, and relatively easy to manage, and knows how to deal with Windows servers as well as standard stuff.
For larger networks, non-Windows-oriented networks, or networks with lots of varied stuff, I heartily recommend OpenNMS. OpenNMS software if free and the company is more than happy to sell support and implementation services. It also happens to be run by a very sharp friend of mine from college!
For those who don't like the Nagios web interface there is NPC, a plugin for Cacti that makes the Nagios UI available from within Cacti, but with better looks (ajax etc.).
It reads from a database provided by NDO2DB, which is a great way to have your infrastructure available from within a database for use in scripts and other tools.
Currently we use PRTG from Paessler. It's excellent. No agents required, excellent Ajax web interface, historical logging, graphing, WMI, etc etc. There's a 10 sensor version available for free but we plonked down a couple of grand for the enterprise version. Money well spent.
Hobbit - it's a faster better version of Big Brother (which seems to be alarmingly commercial these days).
If you're in a hurry and want a quick tool to monitor your MS server then use performance monitor for windows, set up a counter log with custom monitoring template and a custome schedule (eg: collect data for 5 min every hour). Then download Microsoft's LogParser and Codeplex's Performance Analysis of Logs (PAL) Tool (http://pal.codeplex.com/) to crunch your counter log. PAL will generate a great documented report with links to possible issue solving documents/tools.
I use a combination of Solarwinds, VMware server performance tabs, and custom scripts.
Solarwinds Orion Network Performance Monitor is what I use with our Windows sys. admins on my web servers. Still getting some useful app metrics running on it, but it has good information on basic box level stuff (disk, network, CPU).
For my VMware guests, I love the performance tabs.
For my Sun servers, when I need something that isn't available in Solarwinds (because our admin hasn't added it or what), I write custom scripts (usually in Perl) to monitor things like mirror health, swap usage, etc.
I'd like to get more onto Solarwinds, but there's only like 26 hours in a day (or so my boss believes) so I find that can be a tad limiting...
We use OpsView, which runs on top of Nagios. The webUI helps us deploy new host monitor definitions without having to allow SSH access, provides public views, and records historical values. This is handy for provisioning and determining suitable baselines.
Zabbix (http://www.zabbix.com) is good too and easier to setup than Nagios.