CENG 340 Data Management and File Structures

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CENG 340 Data Management and File Structures

• (Fall 2007)

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Introduction to File management
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Motivation

• Most computers are used for data processing (over
  80 billion/year). A big growth area in the
  information age
• This course covers data processing from a
  computer science perspective
• Storage of data
• Organization of data
• Access to data
• Processing of data

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Data Structures vs File Structures

• Both involve
• Representation of Data
• Operations for accessing data
• Difference
• Data structures deal with data in main memory
• File structures deal with data in secondary
  storage

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Where do File Structures fit in Computer Science?
Application
DBMS
File system
Operating System
Hardware
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Computer Architecture
data is manipulated here
- Semiconductors - Fast, expensive, volatile,
small
Main Memory (RAM)
data transfer
Secondary Storage
- disks, tape - Slow,cheap, stable, large
data is stored here
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• Advantages
• Main memory is fast
• Secondary storage is big (because it is cheap)
• Secondary storage is stable (non-volatile) i.e.
  data is not lost during power failures
• Disadvantages
• Main memory is small. Many databases are too
  large to fit in MM.
• Main memory is volatile, i.e. data is lost during
  power failures.
• Secondary storage is slow (10,000 times slower
  than MM)

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How fast is main memory?

• Typical time for getting info from
• Main memory 12 nanosec 120 x 10-9 sec
• Magnetic disks 30 milisec 30 x 10-3 sec
• An analogy keeping same time proportion as above
• Looking at the index of a book 20 sec
• versus
• Going to the library 58 days

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Normal Arrangement

• Secondary storage (SS) provides reliable,
  long-term storage for large volumes of data
• At any given time, we are usually interested in
  only a small portion of the data
• This data is loaded temporarily into main memory,
  where it can be rapidly manipulated and
  processed.
• As our interests shift, data is transferred
  automatically between MM and SS, so the data we
  are focused on is always in MM.

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Goal of the file structures

• Minimize the number of trips to the disk in order
  to get desired information
• Grouping related information so that we are
  likely to get everything we need with only one
  trip to the disk.

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Physical Files and Logical Files

• physical file a collection of bytes stored on a
  disk or tape
• logical file a "channel" (like a telephone line)
  that connects the program to a physical file
• The program (application) sends (or receives)
  bytes to (from) a file through the logical file.
  The program knows nothing about where the bytes
  go (came from).
• The operating system is responsible for
  associating a logical file in a program to a
  physical file in disk or tape. Writing to or
  reading from a file in a program is done through
  the operating system.

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Files

• The physical file has a name, for instance
  myfile.txt
• The logical file has a logical name (a varibale)
  inside the program.
• In C
• FILE outfile
• In C
• fstream outfile

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Basic File Processing Operations

• Opening
• Closing
• Reading
• Writing
• Seeking

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Opening Files

• Opening Files
• links a logical file to a physical file.
• In C
• FILE outfile
• outfile fopen(myfile.txt, w)
• In C
• fstream outfile
• outfile.open(myfile.txt, iosout)

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Closing Files

• Cuts the link between the physical and logical
  files.
• After closing a file, the logical name is free to
  be associated to another physical file.
• Closing a file used for output guarantees
  everything has been written to the physical file.
  (When the file is closed the leftover from the
  buffer is flushed to the file.)
• In C
• fclose(outfile)
• In C
• outfile.close()

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Reading

• Read data from a file and place it in a variable
  inside the program.
• In C
• char c
• FILE infile
• infile fopen(myfile.txt,r)
• fread(c, 1, 1, infile)
• In C
• char c
• fstream infile
• infile.open(myfile.txt,iosin)
• infile gtgt c

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Writing

• Write data from a variable inside the program
  into the file.
• In C
• char c
• FILE outfile
• outfile fopen(mynew.txt,w)
• fwrite(c, 1, 1, outfile)
• In C
• char c
• fstream outfile
• outfile.open(mynew.txt,iosout)
• outfile ltlt c

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Seeking

• Used for direct access an item can be accessed
  by specifying its position in the file.
• In C
• fseek(infile,0, 0) // moves to the beginning
• fseek(infile, 0, 2) // moves to the end
• fseek(infile,-10, 1) //moves 10 bytes from
• //current position
• In C
• infile.seekg(0,iosbeg)
• infile.seekg(0,iosend)
• infile.seekg(-10,ioscur)

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File Systems

• Data is not scattered hither and thither on disk.
• Instead, it is organized into files.
• Files are organized into records.
• Records are organized into fields.

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Example

• A student file may be a collection of student
  records, one record for each student
• Each student record may have several fields, such
  as
• Name
• Address
• Student number
• Gender
• Age
• GPA
• Typically, each record in a file has the same
  fields.

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Properties of Files

• Persistance Data written into a file persists
  after the program stops, so the data can be used
  later.
• Sharability Data stored in files can be shared
  by many programs and users simultaneously.
• Size Data files can be very large. Typically,
  they cannot fit into MM.