File system fragmentation

In computing, file system fragmentation, sometimes called file system aging, is the tendency of a file system to lay out the contents of files non-contiguously to allow in-place modification of their contents. It is a special case of data fragmentation. File system fragmentation increases disk head movement or seek time, which are known to hinder throughput. In addition, file systems cannot sustain unlimited fragmentation. The correction to existing fragmentation is to reorganize files and free space back into contiguous areas, a process called defragmentation.

When a file system is first initialized on a partition, it contains only a few small internal structures and is otherwise one contiguous block of empty space. This means that the file system is able to place newly created files anywhere on the partition. For some time after creation, files can be laid out near-optimally. When the operating system and applications are installed or archives are unpacked, separate files end up occurring sequentially so related files are positioned close to each other.

As existing files are deleted or truncated, new regions of free space are created. When existing files are appended to, it is often impossible to resume the write exactly where the file used to end, as another file may already be allocated there; thus, a new fragment has to be allocated. As time goes on, and the same factors are continuously present, free space as well as frequently appended files tend to fragment more. Shorter regions of free space also mean that the file system is no longer able to allocate new files contiguously, and has to break them into fragments. This is especially true when the file system becomes full and large contiguous regions of free space are unavailable.

The following example is a simplification of an otherwise complicated subject. Consider the following scenario: A new disk has had five files, named A, B, C, D and E, saved continuously and sequentially in that order. Each file is using 10 blocks of space. (Here, the block size is unimportant.) The remainder of the disk space is one free block. Thus, additional files can be created and saved after the file E.

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