 File system check and defragmentation tools typically only account for file fragmentation in their "fragmentation percentage" statistic.Free (unallocated) space fragmentation occurs when there are several unused areas of the file system where new files or metadata can be written to.Unwanted free space fragmentation is generally caused by deletion or truncation of files, but file systems may also intentionally insert fragments ("bubbles") of free space in order to facilitate extending nearby files (see preventing fragmentation below).