What is the origin / meaning of the icon used for "database"?

I've never really understood what this icon, that looks like an oil drum

and is commonly used to refer to a database, is actually supposed to represent.

What is it?

As a matter of fact, I've been searching for any kind of reading material about the history of the symbols commonly used in web app icons and haven't come up with anything helpful or interesting. Is there a good resource to read up on that?