CHAPTER 11: FILE-SYSTEM INTERFACE

1. CHAPTER 11: FILE-SYSTEM INTERFACE • File Concept • File Access • Directory Structure • File System Mounting • File Sharing • File Protection

2. FILE CONCEPT • Introduction • File attributes • File types • File structures • File operations

3. File concept • What is a file • A file is named collection of related information that is recorded on secondary storage. • Content types: • Data • numeric • alphabetic • alphanumeric • binary • Program • source programs • Object forms

4. File concept: Attributes File attributes • Name: The symbolic file name in human-readable form • Identifier: to identify the file within the file system (Internal) • Type: to support different types. • Location: the location on the secondary storage device. • Size: the current size (and/or its maximum size) • Protection: who can do reading, writing, executing, and so on. • Time, date, and user identification: This information may be kept for creation, last modification, and usage monitoring. Typically, the directory entry consists of the file’s name and its unique identifier. The identifier in turn locates the other file attributes. One of the file attributes is the location of file.

5. File concept: Types • If an OS recognizes the type of a file, it can then operate on the file in reasonable ways. • Some common file types (See the next slide) • The TOPS-20 OS: If the user tries to execute an object program whose source file has been modified (or edited) since the object file was produced, the source file will be recompiled automatically. • The Apple Macintosh OS: Each file has a type and also has a creator attribute containing the name of the program that created it. • The Linux: File types are just hints. How to determine a file type on the Linux.

6. File concept: Types

7. File concept: Structure • File structures • None - sequence of words, bytes • Simple record structure • Lines, Fixed length, Variable length • Complex Structures • Formatted document • Relocatable load file • Can be implemented by inserting appropriate control characters. • Who decides: Operating system and Program • Conclusion: Too few structures make programming inconvenient, whereas too many cause OS bloat and programmer confusion.

8. File concept: Operations • Create: to allocate the file space and allocate the directory entry • Read/Write/Seek: to find the directory entry and keep a read/write pointer or seek to a new position. • Delete: to find the directory entry and release all file space and erase the directory entry • Truncate: to find the directory entry and release some file space • Open(F) – to search the directory structure on disk for entry F, and copy the directory entry into the open-file table and allocate a file descriptor. • Close (F) – to copy the directory entry in the open-file table to the directory structure on disk and free the file descriptor

9. File concept: Operations • File tables • Single user: Per-process file table secondary storage • Multiple-users: Per-process file table  system-wide table  secondary storage • Several pieces of information are associated with an open file • File pointer • File open count • Disk location of the file • Access rights

10. FILE ACCESS • Sequential Access read next write next reset • Direct Access read n write/rewrite n position to n read next write next n = relative block number • To simulate sequential access by direct access is easy • (To simulate direct access by sequential access is NOT easy)

11. File access: Sequential-access

12. File access: Direct-access

13. File access: Other access • Index file • Master index  index file  file data (ISAM: Indexed Sequential-Access Method from IBM)

14. File access: Other access

15. DIRECTORY STRUCTURE • Disks  partitions/minidisks/volumes FS  Directories Files • The directory is used to organize files (and directories) • The directory can be viewed as a symbol table that translates file names into their directory entries. • The operations on directories • Search a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system

16. Directory structure A Typical File-system Organization

17. Directory structure: Criteria • 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 • all Java programs • all games • …

18. Directory structures • Single-level directory • Two-level directory • Tree-structured directory • Acyclic graph directory • General graph directory

19. Directory structure: Single-Level Directory • A single directory for all users. • Naming problem • Grouping problem

20. Directory structure: Two-Level Directory • Separate directory for each user. • MFD/UFD/FileName (Path name) • Can have the same file name for different user • Efficient searching • No grouping capability

21. Directory structure: Tree-Structured Directories

22. Directory structure: Tree-Structured Directories • Efficient searching • Grouping Capability • Current directory (working directory) • cd /spell/mail/prog • type list • 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

23. Directory structure: Acyclic-graph directories

24. Directory structure: Acyclic-graph directories • Tree-structured directory + sharing  Acyclic-Graph Directories • Linux ln: • hard link • soft link • Potential problems • To traverse the FS • To delete a file (for symbolic link and hard link)

25. Directory structure: General graph directory

26. Directory structure: General graph directory • Potential problems • To traverse the FS (maybe looping forever) • To determine when a file can be deleted. • Garbage collection

27. FILE SYSTEM MOUNTING • A file system must be mounted before it can be accessed. • A unmounted file system (I.e. Fig. 11-11(b)) is mounted at a mount point.

28. (a) Existing. (b) Unmounted Partition

29. File System Mounting • /etc/fstab • Device • Mount point • File system type • Mount options • For dump • For fsck • Mount/umount, swapon/swapoff • /proc

30. FILE SHARING • For multi-user OS • Sharing of files on multi-user systems is desirable. • Sharing may be done through a protection scheme. • File sharing over the network • FTP • WWW • NFS

31. File Sharing: Remote file systems • The C/S Model • The server: • To declare that a resource is available to clients • To specify exactly which resource and exactly which clients. • The client: • To access the remote files. • The protocol: • Authentication • Operations.

32. File Sharing: Remote file systems • How to use NFS • Startup nfs • /etc/rc.d/init.d/nfs • setup or ntsysv • /etc/exports • Mount NFS • mount server_machine:/exported_fs mount_point • /etc/fstab

33. File Sharing: Remote file systems • Distributed Information Systems • To provide a unified access to the information needed for remote computing. • DNS: domain name to IP address • Yellow pages: to centralize storage of user name, hostnames, printer information, and the like. • Yellow pages  NIS  NIS+ (Sun Microsystems) • Domains  Active directory (Microsoft) • LDAP (Lightweight directory-access protocol)

34. File Sharing: Remote file systems • Failure Modes • For one machine, to use RAID (Redundant Arrays of Inexpensive Disks) to prevent data loss • For networked machines, • Stateful mode: • Too many problems can cause operation to fail. • not good enough. • Stateless • The protocol carries all the information needed to locate the appropriate file and perform the requested operation on a file.

35. File Sharing: Consistency semantics • Consistency semantics: • specifies the the semantics of multiple users accessing a shared file simultaneously. • In particular, these semantics should specify when modifications of data by one user are observable by other users. • UNIX 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

36. File Sharing: Consistency semantics • AFS (Andrew File System) 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. • Immutable-shared file semantics • 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.

37. FILE PROTECTION • File owner/creator should be able to control: • what can be done • by whom • No access  some controlled access  full access • Types of access • Read • Write • Execute • Append • Delete • List

38. File protection • Access control lists • 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.

39. File protection • Chmod • Find • Ls, cd, mkdir, cat, more, less, cmp, diff, sort, merge, tee, touch,

40. Homework • 3, 6, 7, 10, 11