1 File Management Chapter 12

2 File Management n File management system consists of system utility programs that run as privileged applications n Concerned with secondary storage

3 File System Properties n Long-term storage n Sharable between processes n Structure n Operations: u Delete u Open u Close u Read u Write u Create

4 Terms Used with Files n Field u basic element of data u contains a single value u characterized by its length and data type n Record u collection of related fields u treated as a unit F Example: employee record

5 Terms Used with Files n File u collection of similar records u treated as a single entity u have unique file names u may restrict access n Database u collection of related data u relationships exist among elements

6 File Management System n The way a user of application may access files n Programmer does not need to develop file management software

7 Objectives for a File Management System n Meet the data management needs and requirements of the user n Guarantee that the data in the file are valid n Optimize performance n Provide I/O support for a variety of storage device types

8 Objectives for a File Management System n Minimize or eliminate the potential for lost or destroyed data n Provide a standardized set of I/O interface routines n Provide I/O support for multiple users

9 File System Software Architecture Pile Sequential Indexed Sequential Indexed Hashed Logical I/O Basic I/O Supervisor Basic File System Disk Device Driver Tape Device Driver User Program

10 Device Drivers n Lowest level n Communicates directly with peripheral devices n Responsible for starting I/O operations on a device n Processes the completion of an I/O request

11 Basic File System n Physical I/O n Deals with exchanging blocks of data n Concerned with the placement of blocks n Concerned with buffering blocks in main memory

12 Basic I/O Supervisor n Responsible for file I/O initiation and termination n Control structures are maintained n Concerned with scheduling access to optimize performance n Part of the operating system

13 Logical I/O n Allows users and applications to access records n Provides general-purpose record I/O capability n Maintains basic data about file

14 Access Method n Reflect different file structures n Different ways to access and process data

15 Elements of File Management

16 Functions of File Management n Identify and locate a selected file n Use a directory to describe the location of all files plus their attributes n On a shared system describe user access control n Blocking for access to files n Allocate files to free blocks n Manage free storage for available blocks

17 File Directories n Contains information about files u attributes u location u ownership n Directory itself is a file owned by the operating system n Provides mapping between file names and the files themselves

18 Simple Structure for a Directory n List of entries, one for each file n Sequential file with the name of the file serving as the key n Provides no help in organizing the files n Forces user to be careful not to use the same name for two different files

19 Two-level Scheme for a Directory n One directory for each user and a master directory n Master directory contains entry for each user u provides address and access control information n Each user directory is a simple list of files for that user n Still provides no help in structuring collections of files

20 Hierarchical, or Tree-Structured Directory n Master directory with user directories underneath it n Each user directory may have subdirectories and files as entries

Master Directory System User A User B User C Directory “User B’ Directory “User A’ Directory “User C” Draw Word Directory “Word” Unit A Directory “Draw” ABC Directory “Unit A” ABC File “ABC” File “ABC” Pathname: /User B/Word/Unit A/ABC

22 Hierarchical, or Tree-Structured Directory n Files can be located by following a path from the root, or master, directory down various branches u this is the pathname for the file n Can have several files with the same file name as long as they have unique path names

23 Hierarchical, or Tree-Structured Directory n Current directory is the working directory n Files are referenced relative to the working directory

24 File Sharing n Way to control access to a particular file n Users or groups of users are granted certain access rights to a file

25 Access Rights n None u user may not know of the existence of the file u user is not allowed to read the user directory that includes the file n Knowledge u user can only determine that the file exists and who its owner is

26 Access Rights n Execution u the user can load and execute a program but cannot copy it n Reading u the user can read the file for any purpose, including copying and execution n Appending u the user can add data to the file but cannot modify or delete any of the file’s contents

27 Access Rights n Updating u the user can modify, deleted, and add to the file’s data. This includes creating the file, rewriting it, and removing all or part of the data u Changing protection u user can change access rights granted to other users n Deletion u user can delete the file

28 Access Rights n Owners u has all rights previously listed u may grant rights to others using the following classes of users F specific user F user groups F all for public files

29 Simultaneous Access n User may lock entire file when it is to be updated n User may lock the individual records during the update n Mutual exclusion and deadlock are issues for shared access

30 Record Blocking Methods - Fixed Blocking

31 Variable Blocking: Spanned

32 Variable Blocking: Unspanned

33 Secondary Storage Management n Space must be allocated to files n Must keep track of the space available for allocation n Space is allocated as one or more contiguous units or portions

34 Preallocation n Need the maximum size for the file at the time of creation n Difficult to reliably estimate the maximum potential size of the file n Tend to overestimated file size so as not to run out of space

35 Portion Size n Contiguity of space increases performance u Lower seek time u May not have portions that are large enough n Large number of small portions increases the size of tables needed u Better usage of space u Higher seek time n Fixed-size simplifies the reallocation of space n Variable-size minimizes waste of unused storage

36 Methods of File Allocation – Continuous Allocation n Contiguous allocation u single set of blocks is allocated to a file at the time of creation F Idea is similar to arrays u only a single entry in the file allocation table F starting block and length of the file n Lower seek time n Fragmentation will occur n Difficult to find contiguous blocks of sufficient length n Compaction is needed

Contiguous File Allocation File Allocation Table (FAT) File NameStart BlockLength FileA FileB FileC FileD FileE FileA FileB FileC FileE FileD

38 Methods of File Allocation – Chained Allocation n Chained allocation u allocation on basis of individual block u each block contains a pointer to the next block in the chain F Idea is similar to linked lists u only single entry in the file allocation table F starting block and length of file n No fragmentation n Flexible in allocation: Any free block can be added to the chain n No accommodation of the principle of locality u Access time can be high

Chained File Allocation File Allocation Table (FAT) File NameStart BlockLength... FileB FileB

40 Methods of File Allocation – Indexed Allocation n Indexed allocation u file allocation table (FAT) contains a separate one- level index for each file u the index has one entry for each portion allocated to the file u the file allocation table contains block number for the index n Better compromise of performance and space

Indexed Allocation with Block Portions File Allocation Table File NameIndex Block... FileB24 FileB

Indexed Allocation – Variable Length Portions File Allocation Table File NameIndex Block... FileC24 Start BlockLength FileB