File Concept Access Methods Directory and Disk Structure File-System Mounting File Sharing Protection

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2 File Concept Access Methods Directory and Disk Structure File-System Mounting File Sharing Protection

3 File Concepts File Attributes File Operations File Types Internal File Structure

4 A uniform logical view of information storage provided by operating system. A collection of related information. Files are mapped by the operating system onto physical devices. Nonvolatile. A file has a certain defined structure. Text File Object File Executable File

5 Name Identifier Type Location Size Protection Time, date and user identification etc..

6 Creating Writing Reading current file position pointer Repositioning seek Deleting Truncating

7 Etc can be implemented by combination of primitive operations. copy, rename, move. Essential parameter Filename Repetition of constant searching

8 Open-file table Contains information about all open files Open() System Call Parameter : File Name Adds an entry about the file to open-file table Return value : File Handler to an open-file table entry. Close() System Call Parameter : File Handler Remove and entry from open-file table

9 Per-process table Tracks all files that a process has open Each entry in the table has a pointer points to a entry in the system-wide open file table

10 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

11 Open file locking is 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

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13 Disk system typically well-defined block size determined by the size of a sector. All disk I/O is performed in units of one block. Physical record Problem : Logical records may even vary in length. Packing, but it occurs an internal fragmentation

14 Sequential Access Direct Access Other Access Methods

15 Read next / Write next

16 Read block N / Write block N N : Relative block number

17 Index and relative file access method

18 Background Directory Overview Single-Level Directory Two-Level Directory Tree Structured Directories Acyclic-Graph Directories General-Graph Directory

19 A storage device can be partitioned or can be collected together into RAID.

20 A collection of nodes containing information about all files Directory Files F 1 F 2 F 3 F 4 F n

21 Directory operations Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

22 Advantage 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, )

23 Naming problem Grouping problem

24 Path name Can have the same file name for different user Efficient searching No grouping capability

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26 Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog Absolute / Relative path

27 Have shared subdirectories and files

28 Two different names New directory entry type Link another name (pointer) to an existing file Dangling pointer problem Solution Reference Count

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30 Cycle problem occurs. Solution Allow only links to file not subdirectories. Garbage collection Every time a new link is added use a cycle detection algorithm to determine whether it is OK

31 File System Mounting

32 A file system must be mounted before it can be accessed A unmounted file system is mounted at a mount point

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35 Multiple User Remote File System The Client-Server System Distributed Information System Failure Mode Consistency Semantics

36 User IDs identify users, allowing permissions and protections to be peruser Group IDs allow users to be in groups, permitting group access rights

37 Uses networking to allow file system access between systems FTP Distributed File Systems World Wide Web

38 Allows clients to mount remote file systems from servers. Server can serve multiple clients. User-on-client identification is insecure or complicated. Network File System is standard UNIX client-server file sharing protocol. Standard operating system file calls are translated into remote calls.

39 Distributed information systems(distributed naming services) provide unified access to the information needed for remote computing. Sun : Network Information System. Microsoft : Common Internet File System is standard Windows protocol. Lightweight directory-access protocol(ldap)

40 Remote file systems add new failure modes, due to network failure, server failure. Recovery from failure can involve state information about status of each remote request. Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security.

41 Consistency semantics specify how multiple users are to access a shared file simultaneously. Tend to be less complex due to disk I/O and network.

42 Unix file system (UFS) Writes to an open file visible immediately to other users of the same open file. Sharing file pointer to allow multiple users to read and write concurrently. Andrew File System (AFS) Writes to an open file by a user are not visible immediately to other users that have the same file open. 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-Files Semantics

43 Background Types of Access Access Control Other Protection Approaches

44 Issues about keeping information Reliability Protection Reliability Copy integrity problem Protection Need many mechanisms

45 Types of access Read Write Execute Append Delete List

46 Access Control List(ACL)

47 Problem Size of ACL Solution Grouping