ITFT_File system interface in Operating System
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File systems provide structure and organization for files on storage devices. They define file attributes like name, size, permissions and structure directories to group related files in a hierarchical tree. File systems expose files through an abstract interface of operations like create, read, write and delete and maintain metadata about open files and their locations. Access control features like permissions and sharing protocols allow multi-user systems to securely share files.
ITFT_File system interface in Operating System
- 2. File Structure • None - sequence of words, bytes • Simple record structure • Lines • Fixed length • Variable length • Complex Structures • Formatted document • Relocatable load file • Can simulate last two with first method by inserting appropriate control characters • Who decides: • Operating system • Program
- 3. File Attributes • Name – only information kept in human-readable form • Identifier – unique tag (number) identifies file within file system • Type – needed for systems that support different types • Location – pointer to file location on device • Size – current file size • Protection – controls who can do reading, writing, executing • Time, date, and user identification – data for protection, security, and usage monitoring • Information about files are kept in the directory structure, which is maintained on the disk
- 4. File Operations • File is an abstract data type • Create • Write • Read • Reposition within file (seek) • Delete • Truncate • Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory • Close (Fi) – move the content of entry Fi in memory to directory structure on disk
- 5. Open Files • Several pieces of data are needed to manage open files: • File pointer: pointer to last read/write location, per process that has the file open • File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it • Disk location of the file: cache of data access information • Access rights: per-process access mode information
- 6. Open File Locking • Provided by some operating systems and file systems • Mediates access to a file • Mandatory or advisory: • Mandatory – access is denied depending on locks held and requested • Advisory – processes can find status of locks and decide what to do
- 7. File Types – Name, Extension
- 8. Access Methods • Sequential Access read next write next reset no read after last write (rewrite) • Direct Access read n write n position to n read next write next rewrite n n = relative block number
- 10. Directory Structure • A collection of nodes containing information about all files F 1 F 2 F 3 F 4 F n Directory Files Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes
- 11. Disk Structure • Disk can be subdivided into partitions • Disks or partitions can be RAID protected against failure • Disk or partition can be used raw – without a file system, or formatted with a file system • Partitions also known as minidisks, slices • Entity containing file system known as a volume • Each volume containing file system also tracks that file system’s info in device directory or volume table of contents
- 12. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system
- 13. Single-Level Directory • A single directory for all users Naming problem Grouping problem
- 14. Two-Level Directory • Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability
- 16. Tree-Structured Directories (Cont) • Absolute or relative path name • Creating a new file is done in current directory • Delete a file rm <file-name> • Creating a new subdirectory is done in current directory mkdir <dir-name> Example: if in current directory /mail mkdir count mail prog copy prt exp count Deleting “mail” deleting the entire subtree rooted by “mail”
- 17. File Sharing • Sharing of files on multi-user systems is desirable • Sharing may be done through a protection scheme • On distributed systems, files may be shared across a network • Network File System (NFS) is a common distributed file-sharing method
- 18. File Sharing – Multiple Users • User IDs identify users, allowing permissions and protections to be per-user • Group IDs allow users to be in groups, permitting group access rights
- 19. File Sharing – Remote File Systems • Uses networking to allow file system access between systems • Manually via programs like FTP • Automatically, seamlessly using distributed file systems • Semi automatically via the world wide web • Client-server model allows clients to mount remote file systems from servers • Server can serve multiple clients • Client and user-on-client identification is insecure or complicated • NFS is standard UNIX client-server file sharing protocol • CIFS is standard Windows protocol • Standard operating system file calls are translated into remote calls • Distributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing
- 20. Protection • File owner/creator should be able to control: • what can be done • by whom • Types of access • Read • Write • Execute • Append • Delete • List
- 21. Access Lists and Groups • Mode of access: read, write, execute • Three classes of users RWX a) owner access 7 1 1 1 RWX b) group access 6 1 1 0 RWX c) public access 1 0 0 1 • Ask manager to create a group (unique name), say G, and add some users to the group. • For a particular file (say game) or subdirectory, define an appropriate access. owner group public chmod 761 game Attach a group to a file chgrp G game
- 22. Windows XP Access-Control List Management
- 23. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition End of Chapter 10