vmstat

If you post a question to the pgsql-performance mailing list that suggests your system might be overloaded, the first thing you'll be asked for is a snapshot of vmstat data. It's the most valuable quick summary of what your system is doing. Because it displays a full system snapshot per line, it's even possible to extract short-term trends from staring at a screen full of data.

Since the output from vmstat is a bit too wide to fit on the page at once, it's broken up into a left and right side for now; later examples will include just the interesting columns. Here's the left side showing a few seconds of heavy memory-limited pgbench work shown next:

$ vmstat 1
procs -----------memory------------- ---swap--
 r  b   swpd   free    buff   cache   si   so
 8  0      0 2542248 386604 3999148    0    0
 3  0      0 2517448 386668 4023252    0    0
 1  0      0 2494880 386732 4043064    0    0
 7  1      0 2476404 386792 4060776    0    0 

The explanations...