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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 315
284
FILE SYSTEMS
CHAP. 4
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 main-
taining 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 ter-
minate 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, sud-
denly 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.
Linked-List Allocation
The second method for storing files is to keep each one as a linked list of disk
blocks, as shown in Fig. 4-11. The first word of each block is used as a pointer to
the next one. The rest of the block is for data.
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