1. Connecting with Computer Science, 2e
Chapter 10
File Structures 1

2. Objectives In this chapter you will: Learn what a file system does
Understand the FAT file system and its advantages and disadvantages
Understand the NTFS file system and its advantages and disadvantages
Compare common file systems
Learn how sequential and random file access work
See how hashing is used
Understand how hashing algorithms are created
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3. Why You Need to Know About...File Structures
Knowledge of how an operating system stores and maintains data in a computer
Allows better comprehension of how a computer handles and manipulate files
Allows the computer to run as efficiently as possible
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4. What Does a File System Do?
Responsibilities
Creating, manipulating, renaming, copying, and removing files to and from a storage device
Organizing files into common storage units
Called directories
Keeping track of file and directory locations
Assisting users
Relate files and folders to the physical structure of the storage medium
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5. What Does a File System Do? (cont’d.)
Files used by operating systems and applications
Word-processing documents
Source code for programs you have written
Music files
Movie files
Spreadsheets
Photos
Operating systems use a file folder icon to represent a directory
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6. What Does a File System Do? (cont’d.)
Figure 10-1, Files and directories in a file system are similar to documents and folders in a filing cabinet
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7. What Does a File System Do? (cont’d.)
Figure 10-2, Folders and files in Windows
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8. What Does a File System Do? (cont’d.)
Hard disk
Most common storage medium for a file system
Physically organized into tracks and sectors
Read/write heads move over specified areas of the hard disk to store (write) or retrieve (read) data
Random access device
Reads or writes data directly on the disk
Faster than sequential access
Reads and writes from beginning to end
Makes use of the file system to organize files
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9. File Systems and Operating Systems
File management system
Dependent on the operating system
FAT (File Allocation Table)
Used from MS-DOS to Windows ME
NTFS (New Technology File System)
Default for Windows
Unix and Linux support several file systems
XFS, JFS, ReiserFS, ext3, others
Mac OS X file system
HFS and HFS+
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10. FAT Groups hard drive sectors into clusters
Increases performance by organizing blocks of sectors contiguously
Maintains a relationship between files and clusters
Clusters have two entries in the FAT
Current cluster information
Link to next cluster or special code indicating the last cluster
Keeps track of writable clusters and bad clusters
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11. FAT (cont’d.)
Figure 10-3, Sectors are grouped into clusters on a hard disk
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12. FAT (cont’d.) Hard drive organization Partition boot sector
Contains information on how to access volumes
Main and backup FAT
If error in reading the main FAT, backup copied to main to ensure stability
Root directory
Contains entries for every file and folder in the directory
Data area
Measured in clusters
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13. FAT (cont’d.) Figure 10-4, Typical FAT file system
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14. Disk Fragmentation
File clusters scattered in different locations on the storage medium
Windows provides the Disk Defragmenter utility
Reorganizes clusters contiguously
Improves performance
Minimizes movement of the read/write heads
Use regularly to ensure system runs at peak performance
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15. Disk Fragmentation (cont’d.)
Figure 10-5, Files become fragmented as they’re stored in noncontiguous clusters; a defragmenting utility moves files to contiguous clusters and improves disk performance
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16. Advantages of FAT Efficient use of disk space
Does not have to use contiguous space for large files
File names up to 255 characters (FAT32)
Easy to recover deleted files upon deletion
System places E5h in the first position of filename
File remains on drive
Replace E5h with original first letter of the filename
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17. Disadvantages of FAT
Performance slows down as more files are stored on the partition
Hard drive fragments easily
Lack of security
NTFS provides access rights to files and directories
File integrity problems
Lost clusters
Invalid files and directories
Allocation errors
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18. NTFS Overcomes FAT system limitations “Journaling” file system
Keeps track of transaction performed
“Rolls back” transactions if errors found
Uses a Master File Table (MFT)
Stores data about all files and directories
Similar to database table with records
Uses clusters
Reserves blocks of space to allow the MFT to grow
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19. Advantages of NTFS File access is very fast and reliable
MFT allows system recovery from problems without losing significant amounts of data
Security is greatly increased over FAT
File encryption with EFS (Encrypting File System)
File compression reduces file size
Saves disk space
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20. Disadvantages of NTFS Large overhead Cannot access NTFS volumes from:
Not recommended for volumes less than 4 GB
Cannot access NTFS volumes from:
MS-DOS
Windows 95
Windows 98
Linux
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21. Comparing File Systems
Choosing correct file system
Operating system dependent
Rarely depends on hardware
NTFS: Windows XP or Vista
Supports drive sizes up to 16 TB (1600 GB)
FAT: Windows 9x
Older small hard drives, small removable devices
UNIX/Linux
Many file system choices
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22. Comparing File Systems (cont’d.)
Table 10-1, Fat16, FAT32, and NTFS compared
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23. Comparing File Systems (cont’d.)
Table 10-2, Some UNIX/Linux file systems
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24. File Organization Topics covered: File characteristics
How files are stored on disks and other media
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25. Binary or Text Text files Binary files
Consist of ASCII or Unicode characters
Typically read with word-processing programs or text editors
Easy to view and modify
Binary files
Computer readable (not human readable)
Coded and numeric information
More compact than text files
Examples: executable programs, applications, sound and image files
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26. Sequential or Random Access
Sequential storage
Data accessed one chunk after the other in order
Random storage
Data accessed in any order
Also called direct or relative access
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27. Sequential or Random Access (cont’d.)
Figure 10-6, Sequential versus random access
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28. Sequential Access
Starts at the beginning and processes to the end of the file
Writing process is very fast
New data added to the end of a file
Retrieving, inserting, deleting, modifying data
Very slow
Stores data in rows like a database record
Field delimiters or specific fixed sizes for each field
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29. Sequential Access (cont’d.)
Figure 10-7, A comma can be used as a field delimiter
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30. Sequential Access (cont’d.)
Figure 10-8, Data can also be in fixed-length format
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31. Random Access Provides faster access to large amounts of data
Stores fixed-length records (relative records)
Ability to mathematically calculate the record’s position on disk surface and go right to it
Ability to update records in place
May waste disk space
Partial record or no data
Works well when sequential record number can easily identify records
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32. Random Access (cont’d.)
Figure 10-9, Record organization and file access
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33. Hashing Used for accessing relative record files
Uses unique value called a hash key
Widely used in database management systems
Involves a hashing algorithm to generate hash keys for each record
Combining hash keys establishes an index to rows or records of information
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34. Why Hash?
Allows a key field number not suited for relative file access to be converted into a relative record number
Example: phone numbers as keys in a customer information table
Divide highest possible phone number by the expected number of customers to get the hash key
/ 2000 (estimated number of customers) = approximately 5,000,000
Phone number / 5,000,000 gives the record number 1045
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35. Why Hash? (cont’d.) Hashing may result in collisions One solution:
Same relative key is generated for more than one original key value
One solution:
Expand algorithm to add the sum of the digits of the phone number to the relative key
Sum of the digits in phone number is 34
Original key = 1079
Lessens collisions but does not eliminate them
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36. Dealing with Collisions
Best hashing algorithms have collisions
One solution: create overflow area
Records with duplicate record numbers are placed in the overflow area at the end of the file
Record retrieval
Hash key is calculated, and record at calculation position is retrieved
If the record at that location isn’t the correct one, the overflow area is searched sequentially
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37. Dealing with Collisions (cont’d.)
Figure 10-10, An overflow area helps resolve collisions
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38. Hashing and Computing Efficient hashing algorithm
Important to companies producing database management systems
Many different hashing algorithms are used in computing
Encryption and decryption
Indexing
Many programming languages have specialized libraries of built-in hashing routines
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39. One Last Thought Determining a computer system’s worth
Often measured in terms of data stored on hard drives
Data can be difficult to replace
Data storage dependent on file systems
Strong understanding of file systems allows more data availability and protraction
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40. Summary Hard drive File system Windows uses either FAT or NTFS
Random access device
Stores information in tracks and sectors
Accesses data through read/write heads
File system
Responsible for creating, manipulating, renaming, copying, and removing files from a storage device
Windows uses either FAT or NTFS
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41. Summary (cont’d.)
FAT keeps track of which files are using specific clusters
Vulnerable to disk fragmentation
NTFS uses MFT to keep track of files and directories
Used with Windows
NTFS advantages over FAT
Better reliability and security, journaling, file encryption, and file compression
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42. Summary (cont’d.) Linux can be used with many file systems
Files contain binary or text (ASCII) data
Data is usually stored and accessed either sequentially or randomly (relative access)
Hashing
Common method for accessing a relative file
Collisions occur when the hash key is duplicated for more than one relative record location
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