L32_Ch12c

Chapter 12: File Management (Part C: Sections 12.6-12.7)

12.6 Record Blocking

• Definition: Organizing records within blocks on secondary storage.

• Methods of Blocking:

  • 1. Fixed-Length Blocking:

    • Fixed-length records used.

    • An integral number of records stored in a block.

    • Internal Fragmentation: Unused space at end of each block.

  • 2. Variable-Length Spanned Blocking:

    • Variable-length records packed into blocks with no unused space.

  • 3. Variable-Length Unspanned Blocking:

    • Variable-length records used but spanning isn't employed.

    • Fragmentation: A significant concern here.

• Units of I/O: Records organized as blocks for input/output operations.

Illustration

• Figure 12.8: Shows different blocking methods:

  • Red: Fragmentation in Blocks

  • Track representation with various recorded examples.

12.7 File Allocation

• Basics of File Allocation:

  • On secondary storage, a file is a collection of blocks.

  • The system manages allocation of blocks to files.

  • The choice of allocation strategy influences free space management.

• Allocation Structure:

  • Space allocated to files in contiguous portions (blocks).

  • Management through a File Allocation Table (FAT).

Pre-Allocation vs. Dynamic Allocation

• Preallocation:

  • Requires declaration of maximum file size at creation.

  • Tends to be wasteful due to overestimation.

• Dynamic Allocation:

  • Space allocated in portions as needed.

Portion Size Considerations

• Trade-offs:

  1. Contiguity: Increases performance (important for transactions).

  2. Small Portitions: Result in larger tables needed for management.

  3. Fixed Size Portions: Simplifies reallocation of space.

  4. Variable-Size Portions: Minimize wasted storage.

Allocation Alternatives

• Two Major Alternatives:

  1. Variable, Large Contiguous Portions:

     • Better performance with less waste.

  2. Small Fixed Portions:

     • Greater flexibility but may require more complex structures.

File Allocation Table Comparison (Table 12.2)

• Types:

  • Contiguous (Necessary preallocation, Variable portion size).

  • Chained (Possible preallocation, Fixed blocks).

  • Indexed (Possible preallocation, Variable portion size).

Contiguous File Allocation

• Advantages:

  • Efficient for sequential file types.

• Disadvantages:

  • Can lead to fragmentation and unusable short block sequences.

Chained Allocation

• Feature: Can use any free block on disk.

• Disadvantage: Files may be scattered, violating locality.

Indexed Allocation

• Key Point: Distinguish from indexed file types previously covered.

• Indexes: Can accommodate both indexed and non-indexed files.

Free Space Management

• Importance: Managing unallocated space is crucial.

• Disk Allocation Table: Needed alongside File Allocation Table.

Free Space Management Methods

• Bit Tables: Uses a vector indicating free blocks (0 for free, 1 for in use).

  • Advantages: Compact and compatible with all allocation methods.

• Chained Free Portions: Uses pointers to link free portions together.

• Indexing: Treats free space as a pseudo-file with an index.

• Free Block List: Sequential numbering of free blocks maintained.

Volumes

• Definition: Collections of addressable sectors in secondary memory.

• Logical Disk: Presented as a separate disk to users.

• Physical Layout: Need not be physically consecutive; logical continuity maintained.