Chapter 3 part 1


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Chapter 3 part 1

  1. 1. Chapter 3: File Management Part 1
  2. 2. Learning outcome:By the end of this lecture, students will be able to:1) Identify basic function of file system2)Describe file organization techniques3) Describe types of file structure4)Describe various methods of file allocation
  3. 3. File Management : IntroductionFile management system can be define as a system that an operating system uses to keep track of different files.Unlike the CPU and memory management aspects of the operating system which aim mainly towards an optimum use of the CPU, file management aims to provide a convenient programming environment for the users of the system.There are several functions that must be performed by an efficient file system:  include storing of files in an orderly fashion  accessing the stored files  appending the stored files and protecting the files from loss of data
  4. 4. File Structure
  5. 5. File organization techniques Concerned with how records are arranged & characteristics of medium used to store it. On magnetic disks, files can be organized as: 1) Sequential 2) Direct 3) Indexed sequential
  6. 6. Characteristics Considered WhenSelecting 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.
  7. 7. 1) SequentialEasiest to implement because records are stored & retrieved serially, one after other.To speed process some optimization features may be built into system. E.g., 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.
  8. 8. 2) Direct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
  9. 9. Advantages:1) Fast access to records.2)Can be updated more quickly than sequential files because records quickly rewritten to original addresses after modificationsDisadvantage:1) Several records with unique keys may generate same logical address (collision)
  10. 10. 3) Indexed sequential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.
  11. 11. Methods of file allocationFile manager works with files As whole units As logical units or recordsWithin file Records must have same format Record length may varyRecords subdivided into fieldsApplication programs manage record structureThree methods:1) Contiguous file allocation2) Linked list non-contiguous (using blocks)3) Linked list non-contiguous (using index)
  12. 12. 1) Contiguous file allocationRecords stored one after another Advantages  Any record found once starting address, size known  Easy direct access Disadvantages  Difficult file expansion, fragmentation
  13. 13. 2) Linked list non-contiguous(using blocks)Files use any available disk storage spaceFile records stored in contiguous manner If enough empty spaceRemaining file records and additions Stored in other disk sections (extents) Extents  Linked together with pointers  Physical size determined by operating system  Usually 256 bytes
  14. 14. File extents linked in two ways Storage level  Each extent points to next one in sequence  Directory entry  Filename, storage location of first extent, location of last extent, total number of extents (not counting first) Directory level  Each extent listed with physical address, size, pointer to next extent  Null pointer indicates last oneAdvantage Eliminates external storage fragmentation Eliminates need for compactionDisadvantage No direct access support  Cannot determine specific record’s exact location
  15. 15. 3) Linked list non-contiguousallocation (using index) Allows direct record access Brings pointers together  Links every extent file into index block Every file has own index block Disk sector addresses for file Lists entry in order sectors linked Supports sequential and direct access Does not necessarily improve storage space use Larger files experience several index levels