Chapter 13: File-System Interface PDF

Chapter 13: File-System Interface Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018  Outline  File Concept  Access Methods  Disk and Directory Structu...

Chapter 13: File-System Interface Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018  Outline  File Concept  Access Methods  Disk and Directory Structure  Protection  Memory-Mapped Files Operating System Concepts – 10th Edition 13.2 Silberschatz, Galvin and Gagne ©2018  Objectives  To explain the function of file systems  To describe the interfaces to file systems  To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures  To explore file-system protection Operating System Concepts – 10th Edition 13.3 Silberschatz, Galvin and Gagne ©2018  File Concept  Contiguous logical address space  Types: Data  Numeric  Character  Binary Program  Contents defined by file’s creator Many types  text file,  source file,  executable file Operating System Concepts – 10th Edition 13.4 Silberschatz, Galvin and Gagne ©2018  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  Many variations, including extended file attributes such as file checksum  Information kept in the directory structure Operating System Concepts – 10th Edition 13.5 Silberschatz, Galvin and Gagne ©2018  File info Window on Mac OS X Operating System Concepts – 10th Edition 13.6 Silberschatz, Galvin and Gagne ©2018  Directory Structure  A collection of nodes containing information about all files  Both the directory structure and the files reside on disk Operating System Concepts – 10th Edition 13.7 Silberschatz, Galvin and Gagne ©2018  File Operations  Create  Write – at write pointer location  Read – at read pointer location  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 Operating System Concepts – 10th Edition 13.8 Silberschatz, Galvin and Gagne ©2018  Open Files  Several pieces of data are needed to manage open files: Open-file table: tracks 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 Operating System Concepts – 10th Edition 13.9 Silberschatz, Galvin and Gagne ©2018  File Locking  Provided by some operating systems and file systems Similar to reader-writer locks Shared lock similar to reader lock – several processes can acquire concurrently Exclusive lock similar to writer lock  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 Operating System Concepts – 10th Edition 13.10 Silberschatz, Galvin and Gagne ©2018  File Locking Example – Java API import java.io.*; import java.nio.channels.*; public class LockingExample { public static final boolean EXCLUSIVE = false; public static final boolean SHARED = true; public static void main(String arsg[]) throws IOException { FileLock sharedLock = null; FileLock exclusiveLock = null; try { RandomAccessFile raf = new RandomAccessFile("file.txt", "rw"); // get the channel for the file FileChannel ch = raf.getChannel(); // this locks the first half of the file - exclusive exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE); // release the lock exclusiveLock.release(); Operating System Concepts – 10th Edition 13.11 Silberschatz, Galvin and Gagne ©2018  File Locking Example – Java API (Cont.) // this locks the second half of the file - shared sharedLock = ch.lock(raf.length()/2+1, raf.length(), SHARED); // release the lock sharedLock.release(); } catch (java.io.IOException ioe) { System.err.println(ioe); }finally { if (exclusiveLock != null) exclusiveLock.release(); if (sharedLock != null) sharedLock.release(); } } } Operating System Concepts – 10th Edition 13.12 Silberschatz, Galvin and Gagne ©2018  File Types – Name, Extension Operating System Concepts – 10th Edition 13.13 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 13.14 Silberschatz, Galvin and Gagne ©2018  Access Methods  A file is fixed length logical records  Sequential Access  Direct Access  Other Access Methods Operating System Concepts – 10th Edition 13.15 Silberschatz, Galvin and Gagne ©2018  Sequential Access  Operations read next write next Reset no read after last write (rewrite)  Figure Operating System Concepts – 10th Edition 13.16 Silberschatz, Galvin and Gagne ©2018  Direct Access  Operations read n write n position to n read next  write next  rewrite n n = relative block number  Relative block numbers allow OS to decide where file should be placed Operating System Concepts – 10th Edition 13.17 Silberschatz, Galvin and Gagne ©2018  Simulation of Sequential Access on Direct-access File Operating System Concepts – 10th Edition 13.18 Silberschatz, Galvin and Gagne ©2018  Other Access Methods  Can be other access methods built on top of base methods  General involve creation of an index for the file  Keep index in memory for fast determination of location of data to be operated on (consider Universal Produce Code (UPC code) plus record of data about that item)  If the index is too large, create an in-memory index, which an index of a disk index  IBM indexed sequential-access method (ISAM) Small master index, points to disk blocks of secondary index File kept sorted on a defined key All done by the OS  VMS operating system provides index and relative files as another example (see next slide) Operating System Concepts – 10th Edition 13.19 Silberschatz, Galvin and Gagne ©2018  Example of Index and Relative Files Operating System Concepts – 10th Edition 13.20 Silberschatz, Galvin and Gagne ©2018  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 is known as a volume  Each volume containing a file system also tracks that file system’s info in device directory or volume table of contents  In addition to general-purpose file systems there are many special-purpose file systems, frequently all within the same operating system or computer Operating System Concepts – 10th Edition 13.21 Silberschatz, Galvin and Gagne ©2018  A Typical File-system Organization Operating System Concepts – 10th Edition 13.22 Silberschatz, Galvin and Gagne ©2018  Types of File Systems  We mostly talk of general-purpose file systems  But systems frequently have may file systems, some general- and some special- purpose  Consider Solaris has tmpfs – memory-based volatile FS for fast, temporary I/O objfs – interface into kernel memory to get kernel symbols for debugging ctfs – contract file system for managing daemons lofs – loopback file system allows one FS to be accessed in place of another procfs – kernel interface to process structures ufs, zfs – general purpose file systems Operating System Concepts – 10th Edition 13.23 Silberschatz, Galvin and Gagne ©2018  Directory Structure  A collection of nodes containing information about all files  Both the directory structure and the files reside on disk Operating System Concepts – 10th Edition 13.24 Silberschatz, Galvin and Gagne ©2018  Operations Performed on Directory  Search for a file  Create a file  Delete a file  List a directory  Rename a file  Traverse the file system Operating System Concepts – 10th Edition 13.25 Silberschatz, Galvin and Gagne ©2018  Directory Organization The directory is organized 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, …) Operating System Concepts – 10th Edition 13.26 Silberschatz, Galvin and Gagne ©2018  Single-Level Directory  A single directory for all users  Naming problem  Grouping problem Operating System Concepts – 10th Edition 13.27 Silberschatz, Galvin and Gagne ©2018  Two-Level Directory  Separate directory for each user  Path name  Can have the same file name for different user  Efficient searching  No grouping capability Operating System Concepts – 10th Edition 13.28 Silberschatz, Galvin and Gagne ©2018  Tree-Structured Directories Operating System Concepts – 10th Edition 13.29 Silberschatz, Galvin and Gagne ©2018  Acyclic-Graph Directories  Have shared subdirectories and files  Example Operating System Concepts – 10th Edition 13.30 Silberschatz, Galvin and Gagne ©2018  Acyclic-Graph Directories (Cont.)  Two different names (aliasing)  If dict deletes w/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  New directory entry type Link – another name (pointer) to an existing file Resolve the link – follow pointer to locate the file Operating System Concepts – 10th Edition 13.31 Silberschatz, Galvin and Gagne ©2018  General Graph Directory Operating System Concepts – 10th Edition 13.32 Silberschatz, Galvin and Gagne ©2018  General Graph Directory (Cont.)  How do we guarantee no cycles? Allow only links to files not subdirectories Garbage collection Every time a new link is added use a cycle detection algorithm to determine whether it is OK Operating System Concepts – 10th Edition 13.33 Silberschatz, Galvin and Gagne ©2018  Current Directory  Can designate one of the directories as the current (working) directory cd /spell/mail/prog type list  Creating and deleting a file is done in current directory  Example of creating a new file If in current directory is /mail The command mkdir Results in: Deleting “mail”  deleting the entire subtree rooted by “mail” Operating System Concepts – 10th Edition 13.34 Silberschatz, Galvin and Gagne ©2018  Protection  File owner/creator should be able to control: What can be done By whom  Types of access Read Write Execute Append Delete List Operating System Concepts – 10th Edition 13.35 Silberschatz, Galvin and Gagne ©2018  Access Lists and Groups in Unix  Mode of access: read, write, execute  Three classes of users on Unix / Linux RWX a) owner access 7  111 RWX b) group access 6  110 RWX c) public access 1  001  Ask manager to create a group (unique name), say G, and add some users to the group.  For a file (say game) or subdirectory, define an appropriate access.  Attach a group to a file chgrp G game Operating System Concepts – 10th Edition 13.36 Silberschatz, Galvin and Gagne ©2018  A Sample UNIX Directory Listing Operating System Concepts – 10th Edition 13.37 Silberschatz, Galvin and Gagne ©2018  Windows 7 Access-Control List Management Operating System Concepts – 10th Edition 13.38 Silberschatz, Galvin and Gagne ©2018  Memory-Mapped Files Operating System Concepts – 10th Edition 13.39 Silberschatz, Galvin and Gagne ©2018  End of Chapter 13 Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018