Operating system chapter 12

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File Management Chapter 12 

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File Management ° File management system consists of system utility programs that run as privileged applications ° Input to applications is by means of a file ° Output is saved in a file for long-term storage 

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File System Properties ° Long-term existence ° Sharable between processes ° Structure 

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File Operations ° Create ° Delete ° Open ° Close ° Read ° Write 

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Terms Used with Files ° Field ~ Basic element of data ~ Contains a single value ~ Characterized by its length and data type ° Record ~ Collection of related fields ~ Treated as a unit ° Example: employee record 

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Terms Used with Files ° File ~ Collection of similar records ~ Treated as a single entity ~ Have file names ~ May restrict access ° Database ~ Collection of related data ~ Relationships exist among elements 

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Typical Operations ° Retrieve All ° Retrieve One ° Retrieve Next ° Retrieve_Previous ° Insert_One ° Delete _One ° Update One ° Retrieve Few 

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

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Opjectives fora File Management m ° Meet the Byte jement needs and requirements of the user ° Guarantee that the data in the file are valid ° Optimize performance ° Provide I/O support for a variety of storage device types 

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Opjectives fora File Management m ° Minimize System ‏م‎ the potential for lost or destroyed data ° Provide a standardized set of I/O interface routines ° Provide I/O support for multiple users 

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Minimal Set of Requirements ¢ Each user should be able to create, delete, read, write and modify files ° Each user may have controlled access to other users’ files ° Each user may control what type of accesses are allowed to the users’ files ° Each user should be able to restructure the user’s files ina form appropriate to the problem 

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Minimal Set of Requirements ° Each user should be able to move data between files ° Each user should be able to back up and recover the user’s files in case of damage ° Each user should be able to access the user’s files by using symbolic names 

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13 ile System Software Architeeture Figure 12.4 

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

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

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Basic I/O Supervisor ° Responsible for file I/O initiation and termination * Control structures are maintained * Concerned with selection of the device on which file I/O is to be performed * Concerned with scheduling access to optimize performance ° Part of the operating system 

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Logical I/O ° Enables users and applications to access records ۰ Provides general-purpose record I/O capability ° Maintains basic data about file 

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

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0 Figure 12.2 Elements of File Management 19 

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

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Criteria for File Organization ° Short access time ~ Needed when accessing a single record ~ Not needed for batch mode ° Ease of update ~ File on CD-ROM will not be updated, so this is not a concern 

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Criteria for File Organization ° Economy of storage ~ Should be minimum redundancy in the data ~ Redundancy can be used to speed access such as an index ° Simple maintenance ° Reliability 

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File Organization ° The Pile ~ Data are collected in the order they arrive ~ Purpose is to accumulate a mass of data and save it ~ Records may have different fields ~ No structure ~ Record access is by exhaustive search 

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Pile \aiable set of fields Chronological onder (a) Pile Fite 24 

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File Organization ° The Sequential File ~ Fixed format used for records ~ Records are the same length ~ All fields the same (order and length) ~ Field names and lengths are attributes of the file - One field is the key filed * Uniquely identifies the record * Records are stored in key sequence 

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File Organization ° The Sequential File ~ New records are placed in a log file or transaction file ~ Batch update is performed to merge the log file with the master file 

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Sequential File Fized-lengthreconts Fized set of fields in Hxed onder ‘Sequential order based on key Held (b) Sequential Eile 

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File Organization ° Indexed Sequential File ~ Index provides a lookup capability to quickly reach the vicinity of the desired record * Contains key field and a pointer to the main file * Indexed is searched to find highest key value that is equal to or precedes the desired key value ° Search continues in the main file at the location indicated by the pointer 28 

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File Organization * Comparison of sequential and indexed sequential Example: a file contains 1 million records ~ On average 500,00 accesses are required to find a record in a sequential file ~ If an index contains 1000 entries, it will take on average 500 accesses to find the key, followed by 500 accesses in the main file. Now on average it is 1000 accesses 29 

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File Organization ° Indexed Sequential File ~ New records are added to an overflow file ~ Record in main file that precedes it is updated to contain a pointer to the new record ~ The overflow is merged with the main file during a batch update ~ Multiple indexes for the same key field can be set up to increase efficiency 30 

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Indexed Sequential File 

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File Organization ° Indexed File ~ Uses multiple indexes for different key fields ~ May contain an exhaustive index that contains one entry for every record in the main file ~ May contain a partial index 

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Indexed File Exhaustive Exhaustive Partial ‏ها نت‎ index | Primary Fle (variable-length reconts) (d) Indexed File 

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File Organization ° The Direct or Hashed File ~ Directly access a block at a known address ~ Key field required for each record 

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‘Table 12.1 Grades of Performance for Five Basic File Organizations [WIED87] Retrieval Subset Exhaustive D 8 D 5 D B Single record Update Record Size Equal Greater 2 8 D F 8 D 92 90 موه )"00 = Space ‘Auributes File Method | Vasile Fixed A 8 Seaventil F ۸ Indened F B sequential Indexed 8 8 Hashed F Excellent, well suited to this purpose Good Adequate Requires some extra effort Possible with extreme effort ‘Net reasonable for this purpose size of the result umber of records that overflow smmer of records in fle 

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File Directories * Contains information about files ~ Attributes ~ Location ~ Ownership ° Directory itself is a file owned by the operating system ° Provides mapping between file names and the files themselves 36 

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

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Table 12.2 Information Elements of a File Directory File Name ‘Name as chosen by cesior (user or program). Must be unique within a specific directory File Type ‘For example: txt binary, foad module, etc. File Organization For systems that support differest organizations Address Information Volume Indicates device on which eis tored Starting Address Starting physical address on secondary storage (e.evlinder, rack, and block sumber on isk) Sve Used Curent sie of the file i byes, ods, o locks Size Allocated The maximum size ofthe ile 38 

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Access Control Taformation lowner ‘User whe is assigned contol of thi file. The owner may be able to grant de users and to chaage these privileges | Access Information A simple version of this clement would include the user's name and password fore suthorized user lrermitted Actions CConirols reading, writing, executing, transmitting over a network 39 

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Usage Information ‘When file was fist placed in dtectory Usually but not necessarily the enrtent owner Date ofthe lac time a record was ead User who did the reading Date ofthe let update, insrtion, or deletion User who did the modiving Date ofthe lac time the file was backed up on another storage medium Infocmation about curcent activity onthe file, such as process or processes that have the file ‘open, whether itis locked by a process, and whether the file has been updated in main memory bat not vet on diske 40 Date Created Identity of Creator \Date Last Read Access Identity of Last Reader Date Last Modified Identity of Last Modifior Date of Last Backup [Current Usage 

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

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

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Master Directory birectory Subireetory Subireetory > مس Subirectory Subirectory File مس File File File wctured Directory Figure 12.4 Tree-St 43 

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00 وممصم اه عرسي ‎inca‏ تس ۳71 0 1 retoy “Wor” 0 ۳ ۳ ¥ 0 ۰ ¥ 0 ce 1 0 44 igure 12.5 Example of Tree-Structured Directory 

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

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

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File Sharing ° In multiuser system, allow files to be shared among users ° Two issues ~ Access rights ~ Management of simultaneous access 

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Access Rights ۰ None ~ User may not know of the existence of the file ~ User is not allowed to read the user directory that includes the file * Knowledge - User can only determine that the file exists and who its owner is 

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Access Rights ° Execution ~ The user can load and execute a program but cannot copy it ° Reading ~ The user can read the file for any purpose, including copying and execution ° Appending ~ The user can add data to the file but cannot modify or delete any of the file’s contents 49 

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Access Rights ° Updating ~ 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 ° Changing protection ~ User can change access rights granted to other users ° Deletion - User can delete the file 

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Access Rights ° Owners ~ Has all rights previously listed ~ May grant rights to others using the following classes of users * Specific user ° User groups * All for public files 

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

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Fixed Blocking 0 mn ‏ع‎ Ee] be ‏نت‎ ‎0 ۳ a El a 5 Fixed Blocking Data RY Waste due to record fit to block size ۳ caps due to hardware design BB waste due to block size constraint from fixed record size EJ Waste due to block fit to wack size 

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Variable Blocking: Spanned 0 RI 2 RS ll 14 RS RO Track 1 ۳ ۳ ۷ rn ‏مك‎ we fs VY mes ‘Variable Blocking: Spanned Data SQ waste due to record fit wo block size TB cups due to hardware design 959 wae dt back sive contin from fee recon ie ‏نس‎ aw i a 

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Variable Blocking Unspanned RI R2 RB 51 R4 RS N 7 Track 1 R6 1 ۲ RS Ro R10 ‘Track 2 ‘Variable Blocking: Unspanned Data Waste due to record fit to block size ی دسج مج 53 ‎ana‏ ترجه ‎rial seoid ste‏ و ‎‘Waste due to block fit to track size‏ 

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Secondary Storage Management ° Space must be allocated to files ° Must keep track of the space available for allocation 

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

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Methods of File Allocation ° Contiguous allocation ~ Single set of blocks is allocated to a file at the time of creation ~ Only a single entry in the file allocation table * Starting block and length of the file ٠ External fragmentation will occur ~ Need to perform compaction 

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59 Fle Allocation Table eo TE ع 5 ‎ASS‏ لاله ‎s_] ™ s_]‏ ‎oS oa 1‏ ‎we 1‏ ]۱7[ ]16 ]15 ‎7 22 ZAM ZA fla 2 29] ‏حصد ماه نهدي يد‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎Figure 12.7 Contiguous File Allocation ‎ ‎ ‎ ‎ ‎ ‎ 

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60 File Atlocation Table Tile 7 Tiles 9 ۳ 3 File 3 File D 0 File 1 wane ZZ ار a اد uf - ji ZZ) نا" ناه ‎Zw‏ ‏نان لان لاه تعمد ‎ps] of] a]‏ 30] pf] ep] 23 Figure 12.8 Contiguous File Allocation (After Compaction) 

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Methods of File Allocation * Chained allocation ~ Allocation on basis of individual block ~ Each block contains a pointer to the next block in the chain ~ Only single entry in the file allocation table * Starting block and length of file * No external fragmentation ° Best for sequential files * No accommodation of the principle of locality 61 

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62 File Allocation Table Tite Name Start eck Length Files. 1 4 5 لا مس«إماه ماه ناه ماد حا« اللا« اد لاد اعد مب« مد ماك ماه مامد Figure 12.9 Chained Allocation 

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Fite Allocation Table 3 Tile Name Start Rock engi ۰۳۳ www 4 5 oOoooo Files 0 1 ۰ [ ]ده 121 اد ‎ao‏ لحان » اك لاه لان نا« ناه لاه ناه لاد لا« لاه لاد ‎ss‏ ‏اه ]هد ‎sO‏ Figure 12.10 Chained Allocation (After Consolidation) 

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Methods of File Allocation ° Indexed allocation ~ File allocation table contains a separate one-level index for each file ~ The index has one entry for each portion allocated to the file ~ The file allocation table contains block number for the index 

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65 File Allocation Table Tile Name Index Block Breen Figure 12.11 Indexed Allocation with Block Portions 

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File Allocation Table ‘Name Index Block asa iim | 2 esos Figure 12.12 Indexed Allocation with Variable-Length Portions 66 

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UNIX File Management ° Types of files ~ Regular, or ordinary ~ Directory ~ Special ~ Named pipes ~ Links ~ Symbolic links 

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Inodes ° Index node * Control structure that contains key information for a particular file 

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Table 12.4 Information in a UNIX Disk-Resident Inode 16-bit flag that stores access and execution permissions associated with the file, 12-14 File ype (regular, directory, character ot block special, FIFO pie 9-11 Execution flags 8 Owner read permission 7 Owner write permission Orwner execute permission Group read permission Group write permission Group execute permission Other read permission Otter write permission 0 Other execute permission [Number of directory references to this inode Individual owner of file Group owner associated with tis file Namberof bytes in file 39 bytes of address information Time of last file access Time of at file modification Time of last inode modification [File Mode Link Count lowner 1D croup 1۵ Fite Sze lite Addresses Hast Accessed hast Modifica [inode Modifica 

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71 node tate Directory Figure 12.14 UNIX Directories and Inodes 

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Linux Virtual File System * Uniform file system interface to user processes ° Represents any conceivable file system’s general feature and behavior ° Assumes files are objects that share basic properties regardless of the target file system 

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73 ‘Figure 12.15 Linux Virtual File System Context 

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74 stems ge ‏تس‎ rag ‏سس‎ Figure 12.16 Linux Virtual File System Concept, 

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Primary Objects in VFS ° Superblock object ~ Represents a specific mounted file system ° Inode object ~ Represents a specific file ° Dentry object ~ Represents a specific directory entry ° File object ~ Represents an open file associated with a process 

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Windows File System ° Key features of NTFS ~ Recoverability ~ Security ~ Large disks and large files ~ Multiple data streams ~ General indexing facility 

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NTFS Volume and File Structure ° Sector ~ The smallest physical storage unit on the disk ° Cluster ~ One or more contiguous sectors ° Volume ~ Logical partition on a disk 

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Read/write a mirrored or Istriped volume Read/write the disk 1/0 Manager NTFS Driver Fault Tolerant]}¢ Driver Disk Driver Log the transaction 16 و ل } ‎Service‏ ‎Read/write‏ ‎the file‏ Flush the Write the log file cache 1 Cache Load data from cee ree disk into Access the mapped file or flush the cache 1 Virtual ‏مه‎ ‎Manager Figure 12.18 Windows NTFS Components 78