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An analogy for RAM and HDD space

I doubt doubt many readers of this blog will struggle with confusing RAM capacity and Hard Disk capacity. Regardless, I offer this analogy that I use to explain the difference to my less-technical friends in the hope that it will help your less-technical friends understand it:

Think of RAM as your physical wooden-or-formica desktop. It has a certain fixed size and, if you want to work on paper documents, you have to have them on your desk. The number of documents and books you can work on at once is limited by the size of your desk. If you want to work on more paper items at once, get a bigger desk. Similarly, if you want to work with more files and applications at the one time, you need more RAM.

Think if your hard disk as your filing cabinet. It can hold many more paper documents than your desk, but you can’t work on a document while it’s hanging in a drawer. If you want to work on something, you have to move it from the filing cabinet to your desktop. That’s the paper equivalent of loading an application or opening a file. If your desktop is already full, you need to put something back in the filing cabinet.

You keep a lot of things in your filing cabinet that you don’t want to work on every day. Those older documents don’t slow down your work at your desk unless you bring them out and put them on the desk. They can happily live in the filing cabinet forever … until you fill the cabinet and then it’s time to throw away the old stuff or get another cabinet. Similarly, the number of files on your hard drive does not seriously impact the performance of your computer, unless the disk is over 90% full.

There you go. Not drastically insightful but I’ve yet to find another explanation that people of all technical abilities (and, literally, none at all) get as quickly as this one.

In my day, that would have been how our Year 7 IT teacher would have explained it.

‘course, by now, I’d imagine most Year 7s have a pretty good understanding already…

Er, wait. Scottish education system, right? Er, “First year”.

Kids of about that age have a fairly good idea but, in some ways, hard drives are so big now that they don’t often even consider their capacity. I usually talk about iPod capacity when I want to talk about hard drives.

Great analogy. I like the way you’ve illustrated it.

Now, what if I keep a bottle of wine in my filing cabinet. Can I prop my feet on the desk and enjoy a glass with all those books or do I have to flush the desk first?

Making virtual memory like that pile of papers where the stuff you put off sifts to the bottom and can hardly be considered “on top of the desk”, and the drawbacks similar to the time it takes to “page” through the stacks and find something on the bottom.

So Dashboard on Mac OS X is like a few really over-sized desk ornaments.

I have a similar analogy. Keep the desk in your’s, but swap out the file cabinet for a book shelf. If you use a bookshelf in the analogy, then you can explain virtual memory easily too. Say, you have a 4-shelf bookshelf, with only 1 shelf completely full. Virtual memory will take some of the books (open apps) off the table and put them on one of hte empty shelves without closing it - so you don’t lose your place. It works, but it takes longer because you have to ’swap out’ the file. AND disk fragmentation comes about because, imagine that all of your books have no bindings. They get scattered about over time. Defragmenting clears that up and puts things on the shelf in a more orderly fashion.

Thanks for the post!

I’ve been using the exact same analogy myself for years. I wonder if we both heard it somewhere else, or if we independently had the same idea?

I’m thinking exactly the same thing as Stewart - have been using this for ages with only minor modifications. One key question I get hit with is “Well, why does XP/OS X require more RAM than Win98/OS 9″. I usually say that there are some things on the desk that take up space, but which you can’t usefully remove to free up the space underneath - things like the phone, your computer, your printer, lamps etc. I then make the analogy that upgrading your operating system is “like moving from a typewriter to a PC” on your desk. It takes up more room on the desktop when you’re using it, and you can’t get rid of it, but you get more utility from it than before.

Of course, that argument about utility kinda doesn’t work with Vista…