Contiguous allocation can lead to disk external fragmentation, as mentioned in t

Question

Contiguous allocation can lead to disk external fragmentation, as mentioned in the text. Please explain why.

Context: File Systems (Operating Systems)

From the textbook:

   The simplest allocation scheme is to store each file as a contiguous run of disk blocks. Thus on a disk with 1-KB blocks, a 50-KB file would be allocated 50 consecutive blocks. With 2-KB blocks, it would be allocated 25 consecutive blocks.
   We see an example of contiguous storage allocation in Fig. 4-10(a). Here the first 40 disk blocks are shown, starting with block 0 on the left. Initially, the disk was empty. Then a file A, of length four blocks, was written to disk starting at the beginning (block 0). After that a six-block file, B, was written starting right after the end of file A.
   Note that each file begins at the start of a new block, so that if file A was really 3½ blocks, some space is wasted at the end of the last block. In the figure, a total of seven files are shown, each one starting at the block following the end of the previous one. Shading is used just to make it easier to tell the files apart. It has no actual significance in terms of storage.

   Contiguous disk-space allocation has two significant advantages. First, it is simple to implement because keeping track of where a file’s blocks are is reduced to remembering two numbers: the disk address of the first block and the number of blocks in the file. Given the number of the first block, the number of any other block can be found by a simple addition.
   Second, the read performance is excellent because the entire file can be read from the disk in a single operation. Only one seek is needed (to the first block). After that, no more seeks or rotational delays are needed, so data come in at the full bandwidth of the disk. Thus contiguous allocation is simple to implement and has high performance.
   Unfortunately, contiguous allocation also has a very serious drawback: over the course of time, the disk becomes fragmented. To see how this comes about, examine Fig. 4-10(b). Here two files, D and F, have been removed. When a file is removed, its blocks are naturally freed, leaving a run of free blocks on the disk. The disk is not compacted on the spot to squeeze out the hole, since that would involve copying all the blocks following the hole, potentially millions of blocks, which would take hours or even days with large disks. As a result, the disk ultimately consists of files and holes, as illustrated in the figure.
   Initially, this fragmentation is not a problem, since each new file can be written at the end of disk, following the previous one. However, eventually the disk will fill up and it will become necessary to either compact the disk, which is prohibitively expensive, or to reuse the free space in the holes. Reusing the space requires maintaining a list of holes, which is doable. However, when a new file is to be created, it is necessary to know its final size in order to choose a hole of the correct size to place it in.
   Imagine the consequences of such a design. The user starts a word processor in order to create a document. The first thing the program asks is how many bytes the final document will be. The question must be answered or the program will not continue. If the number given ultimately proves too small, the program has to terminate prematurely because the disk hole is full and there is no place to put the rest of the file. If the user tries to avoid this problem by giving an unrealistically large number as the final size, say, 1 GB, the editor may be unable to find such a large hole and announce that the file cannot be created. Of course, the user would be free to start the program again and say 500 MB this time, and so on until a suitable hole was located. Still, this scheme is not likely to lead to happy users.
   However, there is one situation in which contiguous allocation is feasible and, in fact, still used: on CD-ROMs. Here all the file sizes are known in advance and will never change during subsequent use of the CD-ROM file system.
   The situation with DVDs is a bit more complicated. In principle, a 90-min movie could be encoded as a single file of length about 4.5 GB, but the file system used, UDF (Universal Disk Format), uses a 30-bit number to represent file length, which limits files to 1 GB. As a consequence, DVD movies are generally stored as three or four 1-GB files, each of which is contiguous. These physical pieces of the single logical file (the movie) are called extents.
   As we mentioned in Chap. 1, history often repeats itself in computer science as new generations of technology occur. Contiguous allocation was actually used on magnetic-disk file systems years ago due to its simplicity and high performance (user friendliness did not count for much then). Then the idea was dropped due to the nuisance of having to specify final file size at file-creation time. But with the advent of CD-ROMs, DVDs, Blu-rays, and other write-once optical media, suddenly contiguous files were a good idea again. It is thus important to study old systems and ideas that were conceptually clean and simple because they may be applicable to future systems in surprising ways.

Explanation / Answer

Since the wasted storage is between the allocation units not inside them, this is external fragmentation. It is precisely analogous to the external fragmentation of main memory that occurs with a swapping system or a system using pure segmentation.

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