Operating Systems

1. Operating Systems Prof. Navneet Goyal Department of Computer Science & Information Systems BITS, Pilani

2. Topics • Files • Access Methods • Directory Structure • File System mounting • File sharing • Protection

3. Files • Most visible aspect of an OS • Files provide mechanism for online storage of and access to both data and programs of the OS and all the users • A typical file system consists of: • Files (each storing related data) • Metadata (directory structure)

4. Files • Information can be stored on various storage media • OS provides a uniform logical view of information storage • OS abstracts from the physical properties of its storage devices to define a logical storage unit, FILE! • Files are mapped by the OS onto physical devices

5. Files • File is a named collection of related information that is recorded on a secondary storage • Smallest allotment of logical secondary storage • Data cannot be written on secondary storage unless it is written on a file • File is a very general concept

6. File System • Collection of Files File contains related data • Directory Structure Organizes & provides information about all files in the system • Partitions Separates physically or logically large collections of directories

7. Typical File System

8. File Attributes • Name • Identifier (generally a number) • Type • Location (device: location) • Size (bytes, words or blocks) • Protection (rwx) • Time, date, & user identification (creation, last modification, last use)

9. File Operations • File is an ADT • Must define operations on files • File Operations • Creating • Writing • Reading • Repositioning within file (file seek) • Deleting • Truncating • OS provides system calls for each operation

10. File Operations • Creating • Space in the file system must be found for the file • Entry for the new file must be made in the directory • Writing a file • System call specifying the name of the file and the information to be written. System keeps a pointer to the location where the next write is to take place

11. File Operations • Reading • System call specifying the name of the file and where the next block of the file should be put. • A process is either reading from a file or writing to a file, the current operation location can be kept as per process “current-file-position” pointer • Both read and write ops. Use this same pointer to save space & system complexity • Repositioning within a file • Current file position pointer is repositioned to a given value. Need not involve an actual I/O. Also called as file seek

12. File Operations • Deleting a file • Search the directory for the names file, release all file space, and erase the directory entry • Truncating a file • Erases the content of a file but keeps its attributes

13. File Operations • Other Operations • Appending • Renaming • Copy • Move • open() system call • Must be invoked before a file is used • Open file table is maintained by OS • Create & Delete work with closed files

14. File Types

15. Access Methods • Files contain information • Information must be accessed and read into memory for processing • Information in files can be accessed in several ways • Most systems provide only one method • Some systems provide many

16. Access Methods • Choosing the right one for a particular application is a typical design issue • Sequential Access • Direct Access • Indexes (IBMs ISAM)

17. Directory Structure • Stores information about all files • Resides on secondary storage • Contains file’s name & Identifier • Identifier in turn locates other file attributes • More than 1KB of information for each file • Directory structure size in MBs • Backups of files and directory structure kept on tapes

18. Directory Structure • Disks are split into one or more partitions • Minidisks or Volumes • Each disk contains atleast one partition • Partitions can be larger than a disk • Partitions are “virtual disks” • Partition maintains information about files in “Device Directory”

19. Directory Structure A collection of nodes containing information about all files. Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes

20. Information in a Directory • Name • Type • Address • Current length • Maximum length • Date last accessed • Date last updated • Owner ID • Protection information

21. Directory Operations • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system

22. How to Organize a Directory? Issues • Efficiency – locating a file quickly • Naming – convenient to users • Two users can have same name for different files • The same file can have several different names • Grouping – logical grouping of files by properties (e.g. all Java programs, all games, …) Schemes for defining the logical structure of a directory: • Single level directory • Two level directory • Tree structured directory • Acyclic graph directory • General graph directory

23. Single-Level Directory Single directory for all users: easy to support & understand • All files in a single directory • Naming problem (unique names) • Grouping Problem

24. Two-Level Directory Separate directory for each user • each user has its own UFD (user file dir) • UFDs have similar structure • When a user logs in, the MFD (master file dir) is searched • MFD is indexed by user name or accnt no. • Each entry in MFD points to a UFD • User name & a file name define a path name

25. Two-Level Directory Separate directory for each user • When a user refers to a file, only his own UFD is searched • File names can be shared among users • Solves the name-collision problem • Disadvantages? • System files?

26. Two-Level Directory Separate directory for each user • File name would be searched in the current UFD only • Copy system files to each UFD!! • Standard solution: special user directory is defined to contain system files: say user 0 • Search procedure?

27. Tree-Structured Directory

28. Tree-Structured Directory • Generalization of two-level directory (with arbitrary height) • Each user has a current directory (working directory) • Can change current directory via cd command or system call • Path names can be absolute or relative Operations • 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> Advantages • Efficient searching • Grouping Capability

29. Acyclic-Graph Directory • 2 programmers working on a joint project • File pertaining to project stored in a subdirectory of each user • Shared subdirectory! • Tree structure prohibits the sharing of files or directories! • A shared file or dir will exist in the file system in two or more places at once • AGD allows files & dirs to be shared

30. Acyclic-Graph Directory • Shared file is not the same as two copies of the file • With a shared file, only one actual file exists • Shared subdirectories • Files created in this SD will auto. appear in all the shared SDs • LINKS!!

31. Acyclic-Graph Directory • A link is a pointer to another file or subdirectory • Link may be implemented as an absolute or a relative path • When a reference to a file is made • directory is searched • If directory entry is marked as a link, name of the real file is included in the link information • Link is resolved using the path name to locate the real file • Tree traversal is used to traverse directories

32. Acyclic-Graph Directory Duplication • Duplicate all information in both the sharing directories • Original & copy indistinguishable • Maintaining consistency when a file is modified is a problem

33. Acyclic-Graph Directory Problems • AGD is more flexible but more complex • File may have multiple absolute path names. Different file names refer to the same file!!

34. Acyclic-Graph Directory • Have shared subdirectories • and files using acyclic graph • Two different names exist • (aliasing problem) • If ‘dict’ deletes list ⇒ • dangling pointer (UNIX case) • Solutions: • Preserve file until all references to it are deleted • Reference count solution

35. General-Graph Directory • Add links to an existing tree structured directory, resulting in a simple graph structure • Want to avoid searching any shared components twice • Reference count solution does not hold due to self-reference • => use garbage collection • Acyclic structure is much easier to work with • How do we guarantee no cycle? • Allow links only to file not subdirectories • Every time a new link is added use a cycle detection algorithm

36. General Graph Directory • Problem with AGD: ensuring no cycles • Allowing users to create files & directories to an existing tree structure preserves the tree-structured nature • But, when we allow links to be added to an existing tree structure, the tree structure is destroyed, resulting in a simple graph structure

37. File System Mounting • Just as a file must be opened before it is used, a file system must be mounted before it can be accessed • Procedure?

38. File System Mounting Procedure • The OS is given the name of the device, and the location within the file structure at which to attach the file system (or mount point) • Typically, a mount point is an empty directory • The OS verifies that the device contains a valid file system by asking the device driver to read the directory • Finally, the OS notes in its directory structure that a file system is mounted at the specified mount point; This scheme enables the OS to traverse its directory structure, switching among file systems as appropriate

39. File System Mounting

40. 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

41. 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

42. 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

43. Protection • File owner/creator should be able to control: • what can be done • by whom • Types of access • Read • Write • Execute • Append • Delete • List

44. Access Control List (ACL) • Various users may need different types of access to a file or directory • The most general scheme is to associate with each file & directory an access-control list (ACL) specifying the user name & the types of access allowed for each user • Advantages • Enable complex access methodology • Disadvantages • Constructing such a list may be a tedious and unrewarding task, especially if we do not know in advance the list of users in the system • The directory entry, previously of fixed size, now needs to be of variable size, resulting in more complicated space management

45. Access Lists & Groups (UNIX) • Various users may need different types of access to a file or directory • The most general scheme is to associate with each file & directory an access-control list (ACL) specifying the user name & the types of access allowed for each user • Advantages • Enable complex access methodology • Disadvantages • Constructing such a list may be a tedious and unrewarding task, especially if we do not know in advance the list of users in the system • The directory entry, previously of fixed size, now needs to be of variable size, resulting in more complicated space management

46. Access Lists & Groups (UNIX)

47. Sample Directory Listing (UNIX)

48. Windows XP Access-control List Management

49. UNIX File System

50. Working Directory