Internal and External Fragmentation are debated concepts for designing efficient filesystem architectures. Since, fragmentation introduces wastage in usable space. This article discusses the important qualities in both types of fragmentations and why we must consider them.
External Fragmentation is the more commonly known of the two types of fragmentations. Often, inefficient use of memory or hard drive space is due to external fragmentation.
Let us consider an example. Suppose a filesystem allocates files in contiguous blocks. If there is some gap in the filesystem that is too small for any new request, what we have is external fragmentation. In other words, external fragmentation is available space that cannot be used.
Internal Fragmentation is similar to external fragmentation, except that wasted space is within each block of memory (node of a file) that cannot be used.