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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 293
262
MEMORY MANAGEMENT
CHAP. 3
(a) Describe the basic data structures and algorithms in your implementation.
b) Show that your simulation behaves as expected for a simple (but nontrivial) input
example.
(c) Plot the number of TLB updates per 1000 references.
Page 294
4
FILE SYSTEMS
All computer applications need to store and retrieve information. While a proc-
ess is running, it can store a limited amount of information within its own address
space. However, the storage capacity is restricted to the size of the virtual address
space. For some applications this size is adequate, but for others, such as airline
reservations, banking, or corporate record keeping, it is far too small.
A second problem with keeping information within a process’ address space is
that when the process terminates, the information is lost. For many applications
(e.g., for databases), the information must be retained for weeks, months, or even
forever. Having it vanish when the process using it terminates is unacceptable.
Furthermore, it must not go away when a computer crash kills the process.
A third problem is that it is frequently necessary for multiple processes to ac-
cess (parts of) the information at the same time.
If we have an online telephone di-
rectory stored inside the address space of a single process, only that process can
access it. The way to solve this problem is to make the information itself indepen-
dent of any one process.
Thus, we have three essential requirements for long-term information storage:
1.
It must be possible to store a very large amount of information.
2.
The information must survive the termination of the process using it.
3.
Multiple processes must be able to access the information at once.
Magnetic disks have been used for years for this long-term storage.
In recent
years, solid-state drives have become increasingly popular, as they do not have any
263
Page 295
264
FILE SYSTEMS
CHAP. 4
moving parts that may break.
Also, they offer fast random access. Tapes and opti-
cal disks have also been used extensively, but they have much lower performance
and are typically used for backups.
We will study disks more in Chap. 5, but for
the moment, it is sufficient to think of a disk as a linear sequence of fixed-size
blocks and supporting two operations:
1. Read block
k
.
2. Write block
k
In reality there are more, but with these two operations one could, in principle,
solve the long-term storage problem.
However, these are very inconvenient operations, especially on large systems
used by many applications and possibly multiple users (e.g., on a server). Just a
few of the questions that quickly arise are:
1. How do you find information?
2.
How do you keep one user from reading another user’s data?
3.
How do you know which blocks are free?
and there are many more.
Just as we saw how the operating system abstracted away the concept of the
processor to create the abstraction of a process and how it abstracted away the con-
cept of physical memory to offer processes (virtual) address spaces, we can solve
this problem with a new abstraction: the file. Together, the abstractions of proc-
esses (and threads), address spaces, and files are the most important concepts relat-
ing to operating systems.
If you really understand these three concepts from begin-
ning to end, you are well on your way to becoming an operating systems expert.
Files
are logical units of information created by processes.
A disk will usually
contain thousands or even millions of them, each one independent of the others.
In
fact, if you think of each file as a kind of address space, you are not that far off, ex-
cept that they are used to model the disk instead of modeling the RAM.
Processes can read existing files and create new ones if need be. Information
stored in files must be
persistent
, that is, not be affected by process creation and
termination. A file should disappear only when its owner explicitly removes it.
Although operations for reading and writing files are the most common ones, there
exist many others, some of which we will examine below.
Files are managed by the operating system. How they are structured, named,
accessed, used, protected, implemented, and managed are major topics in operating
system design.
As a whole, that part of the operating system dealing with files is
known as the
file system
and is the subject of this chapter.
From the user’s standpoint, the most important aspect of a file system is how it
appears, in other words, what constitutes a file, how files are named and protected,
what operations are allowed on files, and so on. The details of whether linked lists
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