Chapter Eight : File Management

1. The File Manager Interacting With File Manager File Organization Physical Storage Allocation Data Compression Access Methods Levels in File Management System Access Control Verification Module Fixed Length Contiguous Records Storage Non-contiguous Storage Variable Length Records Indexed Storage Sequential or Direct File Access Chapter Eight :File Management

2. Responsibilities of File Manager • Track where each file is stored. • Determine where and how files will be stored. • Efficiently use available storage space. • Provide efficient access to files. • Allocate each file when a user has been cleared for access to it, then record its use. • Deallocate file when it is returned to storage. • Communicate its availability to others waiting for it.

3. Interacting With File Manager • Users communicates with File Manager via specific commands that may be either embedded in user’s program or submitted interactively by user. • Embedded commands: • OPEN & CLOSE pertain to availability of file for program invoking it. • READ & WRITE are I/O commands. • MODIFY – specialized WRITE command for existing data files that allows for appending/rewriting records.

4. Interactive Commands • CREATE & DELETE -- deal with system’s knowledge of file. • SAVE -- first time used, a file is actually created. • OPEN NEW -- within a program indicates file must be created. • OPEN…FOR OUTPUT -- creates file by making entry for it in directory & finding space for it in secondary storage. • RENAME -- allows users to change name of existing file. • COPY – allows user to make duplicate copies of existing files.

5. Commands Are Device-Independent • Interface commands designed to be as simple as possible to use. • Lack detailed instructions to run device where file is stored. • Device independent. • To access a file, user doesn’t need to know its exact physical location on disk pack or storage medium. • Each logical command broken down into sequence of low-level signals that • Trigger step-by-step actions performed by device. • Supervise progress of operation by testing device’s status.

6. Master File Directory (MFD) • MFD stored immediately after volume descriptor • Lists names & characteristics of every file contained in volume. • File names refer to program files, data files, and/or system files. • Subdirectories, if supported. • Remainder of volume is used for file storage. • Early OS supported only a single directory per volume. • Created by File Manager. • Contains names of files, usually organized in alphabetical, spatial, or chronological order. • Simple to implement and maintain. • Some major disadvantages

7. Creation Date Date when volume was created Pointer to Directory Area Indicates first sector where directory is stored Pointer to File Area Indicates first sector where file is stored File System Code Used to detect volumes with incorrect formats Volume News User-allocated name Volume Descriptor

8. Some Major Disadvantages of Single Directory Per Volume • Takes long time to search for an individual file, especially if MFD was organized in an arbitrary order. • If user has many small files stored in volume, directory space fills before disk storage space fills. User told “disk full” when only directory full. • Users can’t create subdirectories to group related files. • Multiple users can’t safeguard files from other users browsing file lists ‘cause entire directory listed on request. • Each program in entire directory needs unique name. • Only 1 person using directory can name program PROG1.

9. About Subdirectories • Semi-sophisticated File Managers create MFD for each volume with entries for files & subdirectories. • Subdirectory created when user opens account to access computer. • MFD entry flagged to indicate subdirectory with unique properties. • Improvement from single directory scheme. • Still can’t group files in a logical order to improve accessibility & efficiency of system.

10. Subdirectories Can Be Implemented As an Upside-down Tree • Today’s File Managers allow users to create subdirectories so related files are grouped together. • Extension of previous two-level directory structure. • Tree structures allow system to efficiently search individual directories due to fewer entries in each. • Path to requested file may lead through several directories. • When user wants to access specific file, file name is sent to File Manager. File Manager searches MFD for user's directory. Then searches user's directory & any subdirectories for requested file & location.

11. File Descriptor Each file entry in every directory contains info describing file: • File name—usually represented in ASCII code. • File type?—organization and usage that are dependent on system (e.g., Files and directories). • File size—size is kept here for convenience. • File location—identification of first physical block (or all blocks) where file is stored. • Date and time of creation. • Owner. • Protection information—access restrictions based on who is allowed to access file and what type of access is allowed. • Record size? —its fixed size or its maximum size, depending on type of record

12. File Names • Absolute file name(complete file name) – long name that includes all path info. • Relative file name – short name seen in directory listings. • Selected by user when file is created. • ACCOUNT ADDRESSES, TAXES 2001, or AUTOEXEC. • Extension – 2-3 character name used to identify type of file or its contents. • Separated from relative name by a period. • CPP, BAS, BAT, COB, & EXE signal to system to use specific compiler or program to run these files. • TXT, DOC, OUT, MIC, & KEY created by applications or by users for own identification.

13. File Naming Conventions • Can vary in length from 1 or more characters. • Can include letters of alphabet & digits. • Every OS has specific rules that affect length of relative name & types of characters allowed. • MS-DOS allows 1-8 alphanumeric character names without spaces. • More modern OS allow names with dozens of characters including spaces BE CAREFUL. • Try to select descriptive relative names that readily identify file contents/purpose of file.

14. Base and Current Directories Used by File Manager to Locate Files • File Manager selects base directory for user when interactive session begins. • All file operations requested by that user start here. • Then, user selects subdirectory (current directory or working directory). • Thereafter, files presumed to be located in current directory. • Whenever file accessed, user types in relative name & File Manager adds proper prefix. • As long as users refer to files in working directory, can access them without entering complete name.

15. File Organization : Record Format • Fixed-length records – easiest to access directly. • Most common type & ideal for data files. • Record size critical (too small – truncation; too large – wastes space). • Variable-length records -- difficult to access directly because hard to calculate exactly where record is located. • Don’t leave empty storage space & don’t truncate any characters. • Frequently used in files accessed sequentially (e.g,. text files, program files) or files using index to access records. • File descriptor stores record format, how it’s blocked, & other related info.

16. Physical File Organization • Concerned with how records are arranged & characteristics of medium used to store it. • On magnetic disks, files can be organized as: • Sequential • Direct • Indexed sequential.

17. Characteristics Considered When Selecting File Organization • Volatility of data—frequency with which additions & deletions made. • Activity of file—% records processed during a given run. • Size of file. • Response time—amount of time user is willing to wait before requested operation is completed.

18. Sequential Record Organization • Easiest to implement because records are stored & retrieved serially, one after other. • To speed process some optimization features may be built into system. • select a key field from record & then sort records by that field before storing them. • Aids search process. • Complicates maintenance algorithms because original order must be preserved every time records added or deleted.

19. Direct Record Organization (Random Organization) • Uses direct access files which can be implemented only on direct access storage devices. • Give users flexibility of accessing any record in any order without having to begin search from beginning of file. • Records are identified by their relative addresses (their addresses relative to beginning of file). • Logical addresses computed when records are stored & again when records are retrieved. • Use hashing algorithms.

20. Advantages of Direct Access Organization • Fast access to records. • Can be accessed sequentially by starting at first relative address & incrementing it by one to get to next record. • Can be updated more quickly than sequential files because records quickly rewritten to original addresses after modifications. • No need to preserve order of the records, so adding or deleting them takes very little time.

21. Collisions Are a Problem With Direct Access Organization • Several records with unique keys may generate same logical address (collision). • Program generates another logical address before presenting it to File Manager for storage. • Colliding records stored in overflow area via links. • File Manager handles physical allocation of space. • Maximum file size established when created & eventually file is full or too many records are stored in overflow area. • Programmer must reorganize & rewrite file.

22. Indexed Sequential Record Organization • Combines best of sequential & direct access. • Created & maintained through Indexed Sequential Access Method (ISAM) software package. • Doesn’t create collisions because it doesn’t use result of hashing algorithm to generate a record’s address. • Uses info to generate index file through which records retrieved. • Divides ordered sequential file into blocks of equal size. • Size determined by File Manager to take advantage of physical storage devices & to optimize retrieval strategies. • Each entry in index file contains highest record key & physical location of data block where this record, & records with smaller keys, are stored.

23. Indexed Sequential - 2 • To access any record in file, system begins by searching index file & then goes to physical location indicated at that entry. • Overflow areas are spread throughout file • Existing records can expand & new records are in close physical & logical sequence. • Last-resort overflow area is located apart from main data area but is used only when the other overflow areas are completely filled. • When retrieval time becomes too slow, file has to be reorganized.. • Allows both direct access to a few requested records & sequential access to many records for most dynamic files. • A variation of indexed sequential files is B-tree.

24. Physical Storage Allocation • File Manager must work with files not just as whole units but also as logical units or records. • Records within file must have same format but can vary in length. • Records are subdivided into fields. • Structure usually managed by application programs, not OS. • When we talk about file storage, we’re actually referring to record storage .

25. R1 R2 R3 R4 R5 R6 Block # R1 R2 R3 Block 1 Recs 2 R1 R1 R2 R2 Length Length R1 # R2 # R3 Block Block # R1 R1 R2 R2 1 Size Recs. Len. Len. • Unblocked, fixed-length records • Blocked, fixed length records • Unblocked, variable-length records • Unblocked, variable-length records • Blocked, variable-length records

26. Contiguous Storage • Records stored one after other. • Any record can be found & read once starting address & size are known, so directory is very streamlined. • Direct access easy – every part of file is stored in same compact area. • Files can’t be expanded unless there’s empty space available immediately following it. • Room for expansion must be provided when file is created. • Fragmentation occurs (slivers of unused storage space). • Can compact & rearrange files. • Files can’t be accessed while compaction is taking place.

27. Noncontiguous Storage • Allows files to use any storage space available on disk. • File’s records are stored in a contiguous manner if enough empty space. • Any remaining records, & all other additions to file, are stored in other sections of disk (extents). • Linked together with pointers. • Physical size of each extent is determined by OS (256 bytes).

28. Linking File Extents • Linking at storage level – each extent points to next one in sequence. • Directory entry consists of file name, storage location of first extent, location of last extent, & total number of extents, not counting first. • Linking at directory level – each extent listed with its physical address, size, & pointer to next extent. • A null pointer indicates that it's last one. • Eliminate external storage fragmentation & need for compaction. • Don’t support direct access because no easy way to determine exact location of specific record.

29. Indexed Storage • Allows direct record access by bringing pointers linking every extent of that file into index block. • Every file has its own index block (addresses of each disk sector that make up the file) • Lists each entry in same order in which sectors linked . • When a file is created, pointers in index block set to null. • As each sector is filled, pointer set to appropriate sector address. • Address is removed from empty space list & copied into its position in index block.

30. Indexed Storage - 2 • Supports both sequential & direct access. • Doesn’t necessarily improve use of storage space because each file must have index block. • For larger files with more entries, several levels of indexes can be generated. • To find a desired record, File Manager accesses first index (highest level), which points to a second index (lower level), which points to an even lower level index & eventually to data record.

31. Data Compression • Several techniques (3) used to save space in files. • System must be able to distinguish between compressed & uncompressed data. • Trade-off: storage space gained, but processing time lost. • Records with repeated characterscan be abbreviated. • E.g., fixed-length field with short name & many blank characters; replaced with variable-length field & special code to indicate # blanks truncated. ADAMSbbbbbbbbbb  ADAMSb10 300000000  3#8

32. Data Compression: Repeated Terms • Repeated terms compressed by using symbols to represent each of most commonly used words in the database. • E.g., in a university’s student database common words like student, course, teacher, classroom, grade, & department could each be represented with single character.

33. Data Compression : Front-end Compression 3. Front-end compression used for index compression. • For example, student database where the students’ names are kept in alphabetical order could be compressed

34. Access Methods • Access methods dictated by a file’s organization • Most flexibility is allowed with indexed sequential files and least with sequential. • File organized in sequential fashion can support only sequential access to its records, & these records can be of fixed or variable length. • File Manager uses the address of last byte read to access the next sequential record. • Current byte address (CBA) must be updated every time a record is accessed.

35. Sequential Access • For sequential access of fixed-length records, CBA updated by incrementing it by record length (RL), which is constant: CBA = CBA + RL • For sequential access of variable-length records, File Manager adds length of record (RLk) plus number of bytes used to hold record length (N) to CBA. CBA = CBA + N + RLk

36. Direct Access & Fixed-Length Records • If file is organized in direct fashion, accessed easily in direct or sequential order if have fixed-length records. • For direct access with fixed length records, CBA computed directly from record length & desired record number RN (info provided through READ command) minus one: CBA=(RN–1) * RL

37. Direct Access & Variable-Length Records • Virtually impossible to access a record directly because address of desired record can’t be easily computed. • To access a record, File Manager must do sequential search through records. • If File Manager saves address of last record accessed, can do half-sequential read through file. When next request arrives it could search forward from CBA. • Or File Manager can keep table of record numbers & their CBAs. Search table for exact storage location of desired record. • To avoid this problem, many systems force users to have files organized for fixed-length records if want direct access to records.

38. Access of Records in Indexed Sequential File • Accessed either sequentially or directly, • Either CBA computations apply but with one extra step. • Index file must be searched for pointer to block where data stored. • Because index file is smaller, kept in main memory & quick search to locate block where desired record is located. • Block retrieved from secondary storage & beginning byte address of record calculated. • In systems with several levels of indexing, index at each level must be searched before computing CBA. • Entry point to this type of data file is usually through index file.

39. Efficient management of files can’t be separated from efficient management of devices that house them. A wide range of functions must be organized for I/O system to perform efficiently. Each level implemented by using structured & modular programming techniques, which also set up a hierarchy. Basic File System Access Control Module Logical File System Physical File System Device Interface Module Device Levels in a File Management System

40. Basic File System • Highest level module that passes info to logical file system, which notifies physical file system, which works with Device Manager. • Activates access control verification module to verify that this user is permitted to perform this operation with this file.

41. Access Control Verification Module • Any file can be shared. • Saves space & allows for synchronization of data updates. • Improves efficiency of system's resources, because if files are shared in main memory, I/O operations reduced. • However, integrity of each file must be safeguarded • Control over who is allowed to access file and what type of access is permitted. • READ only, WRITE only, EXECUTE only, DELETE only, or some combination.

42. File Access Control Methods • Each file management system has own file access control method. • Access control matrix • Access control lists Most • Capability lists Common Methods • Lockword control.

43. Access Control Matrix • Intuitively appealing & easy to implement. • Works well only for systems with few files & few users. • In matrix each column identifies a user & each row identifies a file. • Intersection of row & column has access rights for that user to that file.

44. Access Control Lists • Modification of access control matrix technique. • Each file is entered in list & contains names of users allowed to access it & type of access permitted. • To shorten list, only those who may use file are named; those denied any access are grouped under global heading such as WORLD. • Or shorten by putting every user into a category: • SYSTEM – system personnel with unlimited access to all files. • OWNER – absolute control over all files created in own account. • GROUP – all users belonging to appropriate group have access. • WORLD – all other users in system; default access types given by File Manager.

45. Access Control List Example

46. Capability Lists • Lists every user and files to which each has access. • Requires less storage space than an access control matrix. • Easier to maintain than an access control list when users are added or deleted from system.

47. Lockword Control • Lockword is similar to a password but protects a single file. • When file created, owner protects it via lockword • Stored in directory but isn’t revealed with directory listing. • User must provide correct lockword to access protected file. • Require smallest amount of storage for file protection. • Can be guessed by hackers or passed on to unauthorized users. • Generally doesn’t control type of access to file. • Anyone who knows lockword can read, write, execute, or delete file.