Do you use a Mac and wonder why you don’t have to defragment your hard drive? You can go back and read my first article on why Windows users must defragment their hard drives to better understand what defragmenting your hard drive actually does and why you must do it. Click here for "What is Defragmenting" article Macintosh computers running OSX 10.2 or newer (this is most likely every Mac user especially you as you would not be able to view this webpage if you were using legacy OSX) use a file system called HFS+. This filing system automatically tries to avoid using recently freed up space when it saves files to your hard drive. The filing system instead looks for a nice, large space on the hard drive to plant your files to avoid fragmenting your files in the first place. In addition to this smart placement, OSX will also automatically move small groups of files to a larger space, defragmenting all the files in these groups as it goes along.
OSX also implements Hot File Adaptive Clustering. HFAC is constantly looking for files on your computer that are frequently accessed but not edited (read only). When it finds these files, it moves them to a spot on the drive that can be quickly accessible. This spot is called the hot zone and is reserved for only these types of files. As always when it moves these files it defragments them along the way.
In addition to all of these safeguards, OSX also automatically scans every file when it is opened and if it contains a certain number of fragments, the system will automatically defragment the file. It is worth noting, however, that these safeguards do fail if you let your hard drive fill up over 90% capacity. All of this is happening unbeknownst to you as you play Diablo III or while you are watching Battle Star Galactica on Netflix.