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On hard drives and other types of data storage systems, tracks and sectors are broken into clusters. The cluster is the smallest unit of storage that is addressable (can be written to or read) on the device. The size of clusters may vary. Often you’ll see sizes of 256 or 512 bytes, but this can vary widely from system to system. Each piece of data stored on a disk requires at least one cluster. So if you have a word processing document that’s only 50 bytes in size it will require an entire cluster to store it, even if the clusters are much greater in size. You can’t put two 50 byte files in the same cluster because the computer (or storing device) would have no way to address them separately. Larger file’s clusters can be scattered among different locations on the hard disk. The clusters associated with a file are kept track of in the hard disk’s file allocation table (FAT). When you save or read a file, the entire file is handled for you and you aren’t aware of the clusters it is stored in. The total number of clusters available on a disk depends on how it was formatted and the addressing system used, or more specifically on the size of the FAT table entry. For example, the FAT-32 system commonly used is a 32 bit addressing system, which allows enough cluster addresses to support up to two terabytes (2000 gigabytes) of data, assuming you have a large enough disk.

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