Page 224 - DCAP103_Principle of operating system

Unit 6: File Management

is associated with all of the user’s processes and threads. When they need to be user readable, Notes
they are translated back to the user name via the user name list. Likewise, group functionality
can be implemented as a system-wide list of group names and group identifiers. Every user
can be in one or more groups, depending upon operating system design decisions. The user’s
group IDS are also included in every associated process and thread. The owner and group
IDS of a given file or directory are stored with the other file attributes. When a user requests
an operation on a file, the user ID can be compared to the owner attribute to determine if the
requesting user is the owner of the file. Likewise, the group IDS can be compared. The result
indicates which permissions are applicable. The system then applies those permissions to the
requested operation, and allows or denies it. The user information within a process can be used
for other purposes as well. One process may attempt to interact with another process, and user
information can dictate the result, based on the design of the operating system. For example, a
process may attempt to terminate, background, or lower the priority of another process. If the
owner of each process is the same, then the command may succeed, or else it may be denied.
It may also be allowed to succeed if it is owned by the privileged user. Many systems have
multiple local file systems, including partitions of a single disk or multiple partitions on multiple
attached disks. In these cases, the ID checking and permission matching are straightforward,
once the file systems are mounted.
The user information within a process can be used for other purposes as
well. One process may attempt to interact with another process, and user
information can dictate the result, based on the design of the operating system.
For example, a process may attempt to terminate, background, or lower the
priority of another process. If the owner of each process is the same, then
the command may succeed, or else it may be denied.
6.7.2 Remote File Systems

The advent of networks allowed communication between remote computers. Networking allows
the sharing of resources spread within a campus or even around the world. One obvious resource
to share is data, in the form of files. Through the evolution of network and file technology, file-
sharing methods have changed. In the first implemented method, users manually transfer files
between machines via programs like ftp. The second major method is a distributed file system
(DFS) in which remote directories are visible from the local machine. In some ways, the third
method, the World Wide Web, is a reversion to the first. A browser is needed to gain access
to the remote files, and separate operations (essentially a wrapper for ftp) are used to transfer
files. ftp is used for both anonymous and authenticated access. Anonymous access allows a user
to transfer files without having an account on the remote system. The World Wide Web uses
anonymous file exchange almost exclusively. DFS involve a much tighter integration between the
machine that is accessing the remote files and the machine providing the files. This integration
adds complexity, which we describe in this section.

How to use operating system concept of Remote File Systems?

6.7.3 The Client-Server Model
Remote file systems allow a computer to mount one or more file systems from one or more
remote machines. In this case, the machine containing the files is the server, and the machine
wanting access to the files is the client. The client-server relationship is common with networked
machines. Generally, the server declares that a resource is available to clients and specifies
exactly which resource and exactly which clients. Files are usually specified on a partition or
subdirectory level. A server can serve multiple clients, and a client can use multiple servers,

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