Disk Defragmenter is a system utility provided by Windows for analyzing local logical drives, further locating and combining fragmented files and folders within blocks (clusters) of hard drive. You also can defrag logical drives from a command line using defrag command.

When we save a file it is divided in small stripes and saved on clusters, concerning the size of a cluster the number of fragments are created, for example to save a file of 16KB we need 4 clusters if the size of cluster to hold data is 4KB. Now it is not fixed that all these stripes would be saved in a contiguous fashion.

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