iowait

If ever there was a good reason for physical, spinning hard disks to die, it would be this:

The most important number in that readout is %iowait, which tells you the relative amount of time that your computer, and by extension you, are waiting for data to be delivered from the hard drive to the processor, to be turned into the light and magic that is a running program.

But think about the other numbers: %user says that the amount of useful work being done by the computer is one quarter of one percent per second of runtime, %idle says that the computer could be working 49.87% harder, if it had data available to it.

This is the primary reason why systems feel sluggish when they are first starting up, because none of the data has been cached yet. When you replace a physical hard disk with a solid state disk, %iowait will go straight to 0%, meaning your processor and yourself are never waiting for a slow mechanical system. The faster your drive, the better it can handle reading a ton of information, the better it can read scattered I/O from all over the drive (IOPs, kids), the faster your computer is going to feel.

Another way to think about it: whenever %iowait is more than 0%, you’re effectively reducing the performance of the CPU by that percentage. i.e. If you have a 2GHz processor running with 0% iowait, it’s more effective than a 3GHz processor running with 50% iowait (effectively a 1.5GHz processor, then) because it’s never waiting for data, it’s happily crunching along.

And amazingly, iowait is a wall-clock kind of problem. When a lot of iowaiting is happening, you, in the real world, are waiting for your computer to respond and to do something for you.

So get out there and buy a solid-state disk already.

(And all you companies out there still refusing to make the miniscule $100 investment in an SSD, because it might make that bottom-shelf, corporate-leased hardware ever so slightly more expensive? Well, just keep on doing that. Because time isn’t money, right?)