5.1.4 Fragmented Free Space

The free space (unused portion) of a disk is often fragmented into pieces. The big question is how many pieces, and the size of each piece.

Fragmentation of free space is important because it affects how new files are created. Afterall, when a new file is created, it has to use existing unused clusters on a disk. When the largest available consecutive free cluster block is less than the size of a file, then a file system has no choice but to separate a file into clusters that are not consecutive.

In reality, a file system often create fragmented files even though a block of enough consecutive clusters is available. This is because files are constructed sequentially. A file system cannot predict the size of a file when it is being created. As a result, a file system can start creating a file at the beginning of a small block of consecutive free clusters. As the file size grows, the block of consecutive free clusters will be used up, and clusters from other tracks will have to be used.

A file system that has many small consecutive blocks of free clusters produces new files that are fragmented. This is why it is important to control the fragmentation of free space.