Understanding Hard Disk Cluster Sizes
Clusters are the smallest units into which a hard disk's space can be divided. A hard disk formatted with the traditional FAT system, found in Windows 95 and an ancient operating system called "DOS," can have no more than 65,536 clusters on each drive or partition. This means that the larger the harddisk, the larger the size of each cluster.
The problem with large clusters is that they result in a lot of wasted disk space. Each cluster can store no more than a single file (or a part of a single file); if a file does not consume an entire cluster, the remaining space is wasted. For example, a 2 GB drive would have a cluster size of 32 KB; a 1 KB file on a disk with a 32 KB cluster size will consume 32 KB of disk space; a 33 KB file on the same drive will consume 64 KB of space, and so on.
The extra 31 KB left over from the 33 KB file is called slack space, and it can't be used by any other files. With thousands of files (especially those tiny shortcuts littered throughout a Windows installation), the amount of wasted slack space on a sizeable hard disk can add up to hundreds of megabytes of wasted space.
The NTFS filesystem used by Vista can handle more than four billion clusters, resulting in much smaller cluster sizes. Now, four billion clusters, at 4 kilobytes each, gives NTFS a maximum partition size of 14.9 terabytes (15,259 GB). Of course, if this drive were commercially available, its manufacturer would contend that 1 terabyte is equal to 1,000,000,000,000 bytes, and market the unit as a 16.4 TB (16,384 GB) drive.
You can see how much space is wasted by any given file by right-clicking on the file icon, selecting Properties, and comparing the Size value with the Size on disk value. The same works for multiple selected files and folders; highlight all the objects in your root directory to see the total amount of wasted space on your drive. To find the current cluster size of your drive, just open the properties sheet for a small file you know will only consume a single cluster (such as a Windows Shortcut); its Size on disk will be equal to the size of one cluster.
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