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Why to Do It:

This is one of several Fix-Its that refer to topics first mentioned in Chapter 2, under the heading "Give Your Macintosh a Tune-Up." At that time, I briefly considered them as preventative maintenance procedures. Here, I look at them in more detail, as specific problem-solving techniques. For this Fix-It, the problem is file fragmentation. Suppose that a 50K file is stored on your disk, tucked between two 900K files. If the 50K file is deleted, it leaves a small 50K gap between the two larger files. The larger files cannot automatically slide over to fill in the gap. Files can be moved to different physical locations on a disk only when they are copied or modified. (Remember, location refers to the physical area of the disk that the file occupies. This is different from its location on the desktop, which refers to the folder where it resides. It is also different from the area of memory occupied by a file after it is launched.) After you've spent months adding, deleting, and modifying files on your disk, the unused space on your disk may consist mostly of these small gaps.

Now suppose that you want to copy a new 1200K file to your disk but no longer have a single block that large anywhere on the disk. A total of 5000K of unused space may be on the disk, but it is all in blocks smaller than 1200K. By itself, this is not a problem. Fortunately, the Macintosh can divide the physical storage of a file into separate fragments. These fragments, which don't have to be stored near one another on the disk, then fit into the smaller empty blocks. This is called disk fragmentation or (more accurately) file fragmentation. Similarly, existing files on your disk can become more fragmented each time they are modified (such as when you save changes to a document file).


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