Answer: Cluster Sizing

In order to decrease the overhead of data management, many file systems use a cluster-based approach wherein data is grouped together in chunks anywhere from 512 bytes to 64 kilobytes or more.

In file systems where the average file size is very small (say a computer system storing hundreds of thousands of 4 kilobyte files) and the cluster size has been set very large (say 64 kilobytes), a large amount of space will be wasted as the smallest amount of space that can be allocated (64 kilobytes) is radically larger than the size of the majority of files on the system (4 kilobytes). The space wasted in each cluster by the difference between the file size and cluster size is known as slack space.