Question

I wanted to measure my disk throughput using the following command:

dd if=/dev/zero of=/mydir/junkfile bs=4k count=125000

If the junkfile exists, my disk throughput is 6 times smaller than if junkfile does not exist. I have repeated that many times and the results hold. Anybody knows why?

Thanks,

Amir.

Answer 1

In order to minimize disk caching, you need to copy an amount
significantly larger than the amount of memory in your system. 2X the
amount of RAM in your server is a useful amount.

from http://www.westnet.com/~gsmith/content/postgresql/pg-disktesting.htm