Page 227 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes involve several network communications or several disk reads and writes, or both. Systems that
attempt such a full set of functionalities tend to perform poorly. A successful implementation
of complex sharing semantics can be found in the Andrew File System. For the following
discussion, we assume that a series of file accesses (that is, reads and writes) attempted by a
user to the same file is always enclosed between the open and close operations. The series of
accesses between the open and close operations is a file session. To illustrate the concept, we
sketch several prominent examples of consistency semantics.
6.8.1 UNIX Semantics

The UNIX file system uses the following consistency semantics—Writes to an open file by a user
are visible immediately to other users that have this file open at the same time. One mode of
sharing allows users to share the pointer of current location into the file. Thus, the advancing of
the pointer by one user affects all sharing users. Here, a file has a single image that interleaves
all accesses, regardless of their origin. In the UNIx semantics a file is associated with a single
physical image that is accessed as an exclusive resource. Contention for this single image results
in user processes being delayed.

6.8.2 Session Semantics
The Andrew file system (AFS) uses the following consistency semantics: Writes to an open file
by a user are not visible immediately to other users that have the same file open simultaneously.
Once a file is closed, the changes made to it are visible only in sessions starting later. Already
open instances of the file do not reflect these changes. According to these semantics, a file may be
associated temporarily with several (possibly different) images at the same time. Consequently,
multiple users are allowed to perform both read and write accesses concurrently on their image
of the file, without delay. Almost no constraints are enforced on scheduling accesses.

6.8.3 Immutable-Shared-Files Semantics
A unique approach is that of immutable shared files. Once a file is declared as shared by its
creator, it cannot be modified. An immutable file has two key properties: Its name may not be
reused and its contents may not be altered. Thus, the name of an immutable file signifies that
the contents of the file are fixed, rather than the file being a container for variable information.
The implementation of these semantics in a distributed system is simple, because the sharing
is disciplined (read-only).
6.9 Protection Mechanisms

The concept of multiprogramming introduces the sharing resources among users. This sharing
involves Memory, I/O devices, Programs and Data. The ability to share these resources
introduces the need for protection. An OS may offer protection along the following Spectrum:
No Protection: This is appropriate when sensitive procedures are being run at separate times
Isolation. This approach implies that each process operates separately from other processes, with
no sharing. Each process has its own address space, files, and other objects Share all or Share
nothing: In this method, the owner of an object declares it to be public or private, in the other
words, only the owner‘s processes may access the object. Share via access limitation—The OS
checks the permissibility of each access by a specific user to specific object; the OS therefore acts
as a guard between users and objects, ensuring that only authorized accesses occur. Share via
dynamic capabilities: This extends the concept of access control to allow dynamic creation of
sharing rights for objects. Limit use of an object—This form of protection limits not just access to
an object but the use to which that object may be put. A given OS may provide different degree
of protection for different objects, users and applications The OS needs to balance the need to
allow sharing, with the need to protect the resources of individual users.

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