Imagine that you're load testing your servers, or maybe you're monitoring live servers during peak times. You'll probably want to monitor CPU and memory, and maybe some stats of special interest, like database inserts. You might start by logging into the servers, and running something like "top" to monitor activity. Sometimes you want a little more though, and that's where tools such as jLlama and Zenoss come in. Both are Open Source tools capable of graphing server performance statistics, but they work in very different ways.
jLlama is a quick and simple way of running commands on remote servers (over SSH) and graphing the output of these commands. The statistics are stored in a database so they can be read back at any time. jLlama is a stand alone Java application, making it quick and easy to set up and start using right away. Linux and Solaris CPU and memory statistics can be graphed right away, or you can configure it to run your own custom shell commands to graph anything else you want (such as CPU temperature, database queries, network statistics etc).
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Zenoss does much the same kind of thing, although it (usually) uses SNMP to communicate with servers, although other methods such as remote commands are available. It's controlled through a web interface, making it easy to share performance statistics. In comparison to jLlama, Zenoss is much harder to set up and configure, but once it's running it forms an excellent server monitoring tool, capable of collecting statistics from hundreds of servers continuously.
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Cacti is another tool worthy of consideration. It works in a similar way to Zenoss, collecting statistics using SNMP and presenting results through a website. The advantage over Zenoss is in its simplicity, but this might also be its failing if you have complicated requirements. (Ubuntu users: you can install Cacti straight from the package manager!)
Both Zenoss and jLlama are capable of graphing any statistic you can retrieve from the command line of a server, such as CPU usage or memory usage. If you want an "enterprise ready" solution to monitor the same servers day in day out, prepare to spend some quality time getting to know Zenoss. If you want to quickly graph statistics of several servers from one place, jLlama is the tool for you!