Several times in this chapter, I mentioned the swap partition but have yet to give it a formal discussion. Swap is one of those things we never want to use, but always want to make sure is available. There's even some debate between administrators on whether or not swap is still relevant today. It's definitely relevant, regardless of what anyone says, as it's a safety net of sorts.
So what is it? Swap is basically a partition or a file that acts as RAM in situations where your server's memory is saturated. If we manage a server properly, we hope to never need it, as swap partition is stored on your hard disk which is orders of magnitude slower than RAM. But if something goes wrong on your server and your memory usage skyrockets, swap may save you from having your server go down. It's a good idea to have it, and considering that hard drive...