Understanding RAM versus hard-drive space via an analogy
When troubleshooting computers many people suggest "freeing up memory" and similar options carelessly without differentiating between RAM and hard-drive space. Here is a quick analogy to illustrate the difference and how each may affect system performance.
Topher KesslerMacFixIt Editor
Topher, an avid Mac user for the past 15 years, has been a contributing author to MacFixIt since the spring of 2008. One of his passions is troubleshooting Mac problems and making the best use of Macs and Apple hardware at home and in the workplace.
When troubleshooting Macs and other systems, the word "memory" may be thrown around rather carelessly when addressing problems such as slowdowns. People may suggest "freeing up memory," increasing RAM, or doing the same for hard-drive space; however, the similarity and overlapping use of these words can confuse people, especially given that both RAM and hard-drive contents are sized in megabytes and gigabytes.
I regularly receive comments and questions from people asking me about RAM and hard-drive optimizations and including phrases similar to, "I know that the problem has to do with RAM (I have 2GB), but what types of files can I delete to free this up?"
In order to tackle slowdowns that happen because of low system resources, it is good to first understand in principle the ways that RAM (memory) and hard-drive space (storage) will affect performance, and the best way I've found to view this is through the analogy of a work desk.
While RAM may be a factor in slowdowns, RAM is not your storage medium, so you cannot manually delete anything to free it up. RAM instead is a temporary storage location like your desk's work surface, which if it's cluttered with tools, papers, and other items (in other words, open applications or system add-ons) will be harder to organize and use.
Just imagine the desk above containing lamps, pen holders, some pictures, and other "widgets"; each may use up a small amount of space but together they use up lots of space. This would be similar to having multiple system add-ons and tools all running in the background on your system. If you have the space for them then they won't cause a problem, but if you have too many apps open or if your work space is relatively small then you might crowd the rest of your work environment.
To keep this work space on your system free and usable, quit open applications and close files and documents (especially large ones) that you do not need to use. Sometimes a quick system restart is the easiest way to free up RAM. You can also purchase more RAM for your system to increase the size of the computer's work space (similar to getting an extension for the desk), thereby enabling yourself to get more done at the same time without chancing any slowdowns.
Hard drive (storage)
Unlike RAM, which is the work space of the system, the hard drive is like the filing cabinet, drawers, and other storage locations of a desk where tools and papers are stored when not used. If the cabinets of your desk are already nearly full, it will be harder to stuff items in there without moving things around and constantly reorganizing. Therefore, in order to keep your work flow going you can either get another set of drawers or cabinets, or clean out the current ones.
Virtual memory (stored memory)
Virtual memory on a computer can be best compared to a pull-out drawer in a desk that is easy to access but can be pushed out of the way of your main work space, and is used by the computer to temporarily store RAM contents that are loaded but not currently in use.
When the system is low on RAM (work space) the system will make more use of virtual memory to free up the work space and allow other applications to open and load more documents. Even when the system does not need to use a lot of RAM, it still makes use of virtual memory in order to keep the work space tidy and organized.
If there is no place to store virtual memory (unused but active RAM contents) then the system will have to make do with a smaller work surface, which can cause work-flow bottlenecks, cause hangs, and make the spinning color wheel show up. Just imagine the desk above with papers crammed into every drawer and cabinet, so when you try to move something off the work surface there is no place to put it.
Because of this use of hard-drive space to organize RAM contents, it is always good to keep space free on your boot drive so the system can manage virtual memory and make the system run more efficiently. A good rule of thumb is to keep about 10 percent of your hard drive free. Check your installed applications, downloaded movies and music, and other files you have placed on your system to see if any are using up a lot of disk space (gigabytes). If so, try to thin them out a bit by copying them to an external hard drive and then deleting them from your boot drive.
Additional optimization options
Besides freeing up hard-drive space (if needed) and getting more RAM (or at least not keeping as many applications open), try running a general maintenance routine on your system, which can be done by following the instructions here.
A general maintenance routine can help by removing and rebuilding various odd temporary files the system uses, as well as making sure system files are accessible, just like doing a monthly clean-out and organization of your desk.
You can also use a hard-drive optimization tool such as iDefrag or Drive Genius to defragment the drive. This is very similar to going through your file cabinets and organizing your papers so the relevant ones are close together, making them all easier to access when you need to get them.