Chapter-10-OS.pptx-Operating System Course for MSc Computer Science Programme
- 2. 10.4 File-System Mounting • File must be opened before it is used, similarly a file system must be mounted before it can be available to processes on the system. • The directory structure can be built out of multiple volumes, which must be mounted to make them available within the file-system name space. • The mount procedure is straightforward. • The operating system is given the name of the device and the mount point— the location within the file structure where the file system is to be attached.
- 3. • A mount point is an empty directory. • For instance, on a UNIX system, a file system containing a user's home directories might be mounted as /home; then, to access the directory structure within that file system, we could precede the directory names with ftiome, as in /homc/jane. • Mounting that file system under /users would result in the path name /users/jane, which we could use to reach the same directory. • Next, the operating system verifies that the device contains a valid file system. • It does so by asking the device driver to read the device directory and verifying that the directory has the expected format. • Finally, the operating system notes in its directory structure that a file system is mounted at the specified mount point. • This scheme enables the operating system to traverse its directory structure, switching among file systems as appropriate.
- 5. 10.5 File Sharing • The motivation for file sharing and some of the difficulties involved in allowing users to share files. • Such file sharing is very desirable for users who want to collaborate and to reduce the effort required to achieve a computing goal • User-oriented operating systems must accommodate the need to share files in spite of the inherent difficulties. • We begin by discussing general issues that arise when multiple users share files. • Once multiple users are allowed to share files, the challenge is to extend sharing to multiple file systems, including remote file systems; and we discuss that challenge as well. • Finally, we consider what to do about conflicting actions occurring on shared files. • For instance, if multiple users are writing to a file, should all the writes be allowed to occur, or should the operating system protect the user actions from one another?
- 6. 15.1: Multiple Users • Access Control and Protection: • Operating systems with multiple users must handle file sharing, naming, and protection. • Users can access each other’s files by default or require explicit access grants. • Concepts like file owner and group play a crucial role. • Owner and Group Concepts: • The file owner has the most control over the file and can modify attributes. • Group members share access to the file. • Permissions are defined by the file’s owner. • Permission Attributes: • Owner and group IDs are stored with file attributes. • When a user requests an operation, the system compares IDs to determine applicable permissions. • These concepts ensure consistent access control while balancing security and usability in multi-user environments
- 7. • Certainly! Let’s delve into the world of remote file sharing and explore different methods used for accessing files across networks. • Manual File Transfer (FTP): • The earliest method involved manually transferring files between machines using File Transfer Protocol (FTP). • Users would initiate FTP sessions to transfer files from one machine to another. • FTP supports both anonymous access (where users can transfer files without an account) and authenticated access. • The World Wide Web also relies on FTP for anonymous file exchange. • Distributed File Systems (DFS): • DFS provides a more structured approach to remote file sharing. • In DFS, remote directories are visible from a local machine. • It allows seamless integration between the local and remote file systems. • However, managing DFS can be complex due to tight integration. • The World Wide Web (WWW): • The WWW, in some ways, reverts to manual file transfer. • Users access remote files through browsers, which act as wrappers for FTP. • While browsing, separate operations (essentially FTP requests) are used to transfer files. • Client-Server Model: • In remote file systems, a computer can mount file systems from remote machines. • The machine containing the files acts as the server, and the requesting machine is the client. • The server declares available resources (files) and specifies which clients can access them. • Client identification can be challenging, but the client-server relationship is common in networked environments. • Remember that these methods facilitate efficient file sharing across networks, each with its own advantages and complexities12 . Whether it’s manual FTP, structured DFS, or the simplicity of the WWW, remote file access plays a crucial role in today’s interconnected world.
- 8. 15.3: Consistency semantics • It play a crucial role in evaluating file systems that support file sharing. • These semantics define how multiple users can access a shared file simultaneously and specify when modifications made by one user become visible to others
- 9. • UNIX Semantics: • Immediate visibility of modifications to all users. • Shared files have a single physical image, leading to delays. • Session Semantics (Andrew file system): • Changes made by one user are not immediately visible to others. • Only visible to sessions starting later. • Efficient concurrent access without delays. • Immutable-Shared-Files Semantics: • Users cannot modify files declared as shared. • Immutable files have fixed names and content. • These semantics ensure consistent file access across users while balancing functionality and performance .