Chapter 11: File-System Interface. File Concept - PDF

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Chapter 11: File-System Interface File Concept Access Methods Directory Structure File System Mounting File Sharing Protection File Concept Contiguous logical address space Types: Data numeric character
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Chapter 11: File-System Interface File Concept Access Methods Directory Structure File System Mounting File Sharing Protection File Concept Contiguous logical address space Types: Data numeric character binary Program 1 1 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 File Attributes Name only information kept in human-readable form. 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. 2 2 File Operations Create Write Read Reposition within file file seek Delete Truncate Open(F i ) search the directory structure on disk for entry F i, and move the content of entry to memory. Close (F i ) move the content of entry F i in memory to directory structure on disk. File Types Name, Extension 3 3 Access Methods Sequential Access Direct Access n = relative block number read next write next reset no read after last write (rewrite) read n write n position to n read next write next rewrite n Sequential-access File 4 4 Simulation of Sequential Access on a Direct-access File Example of Index and Relative Files 5 5 Directory Structure A collection of nodes containing information about all files. Directory Files F 1 F 2 F 3 F 4 F n Both the directory structure and the files reside on disk. Backups of these two structures are kept on tapes. A Typical File-system Organization 6 6 Information in a Device Directory Name Type Address Current length Maximum length Date last accessed (for archival) Date last updated (for dump) Owner ID (who pays) Protection information (discuss later) Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system 7 7 Organize the Directory (Logically) to Obtain 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, ) Single-Level Directory A single directory for all users. Naming problem Grouping problem 8 8 Two-Level Directory Separate directory for each user. Path name Can have the same file name for different user Efficient searching No grouping capability Tree-Structured Directories 9 9 Tree-Structured Directories (Cont.) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list 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 Acyclic-Graph Directories Have shared subdirectories and files. Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list dangling pointer. Solutions: Backpointers, so we can delete all pointers. Variable size records a problem. Backpointers using a daisy chain organization. Entry-hold-count solution General Graph Directory General Graph Directory (Cont.) How do we guarantee no cycles? 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 File System Mounting A file system must be mounted before it can be accessed. A unmounted file system is mounted at a mount point. (a) Existing. (b) Unmounted Partition 13 13 Mount Point 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 filesharing method Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access RWX b) group access RWX c) public access 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 Attach a group to a file chgrp chmod 761 game G game 15 15
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