What is the difference between compaction and defragmentation?

Question

My operating systems textbook says that compaction is a process that rearranges disk blocks such that all free disk blocks form a contiguous "chunk" of free disk space.

But I always thought that was what defragmentation does? Are these two terms the same? Or am I missing something?

External

Answer 1

Compaction :- means moving the "in-use" memory areas to eliminate holes caused by terminated processes.Suppose we have five processes A, B, C, D, E, allocated as |A|B|C|D|E| in memory. After sometime process B and D are terminated. Now we have memory layout as |A| |C| |E|. After applying compaction we will have |A|C|E| | | i.e instead of two one-block memory unit we have one two-block memory unit.

Defragmentation :- means storing complete file in smallest number of contiguous regions. That is, it tries to store file as one complete unit if that size of contiguous memory is available. Suppose process A has fragments A1, A2, A3, process B has fragments B1, B2. Now, suppose memory layout is |A1|B1|A2|A3|B2|, after defragmentation we have |A1|A2|A3|B1|B2|.

Defragmentation can also contribute to compaction.

Thenna

Answer 2

In modern disk operating systems, files are subdivided into blocks, each of which may be stored at an arbitrary location on the disk. Files can be read most quickly from a physical disk if the blocks are stored consecutively, but I think every OS that was created since the mid-1980's can, without difficulty, create a file which is larger than the single largest consecutive free area on the disk, provided that the total size of all free areas is sufficient to hold the file. Such a file will end up with different pieces stored in different formerly-free parts of the disk, and thus accessing it will often not be as fast as if the entire file had been stored consecutively.

Conceptually, an "ideal" disk arrangement would have the contents of every file stored consecutively, with all files stored "back-to-back", so all the unused blocks were in a consecutive range. Such an arrangement would be both "compacted" and "defragmented". In general, though, the amount of effort to arrange everything perfectly is seldom worthwhile (with an obvious exception being a disk that is written all at once, and will never be modified, as would typically be the case with e.g. a CD-ROM). Defragmenting a disk will move all the blocks that make up each file to a consecutive sequence of blocks on the disk, but will not necessarily attempt to eliminate free areas between files. Compacting a disk will consolidate all of the free areas by moving data from later parts of the disk to unused locations in earlier parts, but may cause fragmentation of existing files.

Generally, software that performs defragmentation will try to avoid creating too many scattered free areas, and software that performs compaction will try to avoid causing needless fragmentation, but depending upon what the software is trying to do (e.g. maximize efficiency for existing files, versus preparing a large contiguous area of space in preparation for a large data-acquisition operation that needs to run smoothly) the software may focus on one kind of operation at the expense of the other.

Borax