Just to clear this one up...
Got to Site admin > Plugins > Caching > Test performance.
Post the results
If you are keen, test your file IO with sysbench (or similar). For example...
sysbench --test=fileio --file-total-size=4G prepare
sysbench --test=fileio --file-total-size=4G --file-test-mode=rndrw run
sysbench --test=fileio --file-total-size=4G cleanup
Here's the results from my test box for comparison. I bang on about it, but file IO speed is critical for the cache and cannot be ignored.
Operations performed: 6000 Read, 4000 Write, 12800 Other = 22800 Total
Read 93.75Mb Written 62.5Mb Total transferred 156.25Mb (5.1863Mb/sec)
331.92 Requests/sec executed
Test execution summary:
total time: 30.1276s
total number of events: 10000
total time taken by event execution: 0.0749
per-request statistics:
min: 0.00ms
avg: 0.01ms
max: 0.05ms
approx. 95 percentile: 0.01ms
Nagios is *usually* used for monitoring critical incidents on the server. We really want to see what things like load and memory use are over time. We use Munin *and* Nagios. When Nagios starts complaining we go and look at the Munin graphs basically.