What is the difference between external and internal fragmentation?

Answer 1

Fragmentation occurs in a dynamic memory allocation system when many of the free blocks are too small to satisfy any request.

External Fragmentation: External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous.

Internal Fragmentation: Internal fragmentation is the space wasted inside of allocated memory blocks because of restriction on the allowed sizes of allocated blocks. Allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used

Another Definition:

Internal Fragmentation is the area in a region or a page that is not used by the job occupying that region or page. This space is unavailable for use by the system until that job is finished and the page or region is released.

Answer 2

To illustrate the difference between internal and external fragmentation, let's consider the system registry or some other database. It can be externally fragmented in that an entire file as a whole is what is fragmented. So the SYSTEM hive of the system registry might be split up into a number of chunks on the hard drive. So regular disk fragmentation is what causes this.

Using the same example, it could also be fragmented internally. The system registry contains many keys, and the keys are constantly being added and deleted. So the registry might have "holes" in it between the entries. These holes make the registry longer than it needs to be. The way to eliminate this is to rebuild the database from scratch. In the specific example of the system registry, there is a program called NTREGOPT which can recreate all the hives, copying only the valid data and leaving the holes or any invalid data behind.

The filesystem itself is a database, and files in it can be fragmented in relation to it. That is external fragmentation. In addition, database files can be stored within that filesystem, and while they can become fragmented in relation to the filesystem, they can also have entries inside in a less than optimal order with spaces from deleted entries between the records.