GATE CSE 2008 | Question: 20

Question

The data blocks of a very large file in the Unix file system are allocated using

• A. continuous allocation
• B. linked allocation
• C. indexed allocation
• D. an extension of indexed allocation

Comments

In the Unix File System (UFS), the data blocks of a very large file are allocated using:

An extension of indexed allocation

Explanation:

1. Indexed Allocation:

   • Unix uses an inode (index node) structure to store information about files. Each inode contains:
     • Metadata (permissions, timestamps, etc.)
     • Pointers to data blocks.
   • The inode has a fixed number of direct pointers, single indirect pointers, double indirect pointers, and triple indirect pointers.

2. Extension of Indexed Allocation:

   • Direct pointers: Point directly to data blocks.
   • Single indirect pointers: Point to a block that contains pointers to data blocks.
   • Double indirect pointers: Point to a block that contains pointers to single indirect blocks.
   • Triple indirect pointers: Point to a block that contains pointers to double indirect blocks.

   This hierarchical approach allows Unix to handle very large files efficiently by extending the indexed allocation mechanism.

Why Not Other Options?

• Continuous Allocation:
  Not used in Unix because it suffers from fragmentation and is inflexible for dynamic file sizes.

• Linked Allocation:
  Not used because it requires sequential access, and random access becomes inefficient.

• Indexed Allocation (Simple):
  Unix uses an extended form of indexed allocation to handle large files, which surpasses the limits of simple indexed allocation.

Thus, the correct answer is "an extension of indexed allocation.

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an extension of indexed allocation or The extended file system, or ext, file system used in Unix OS.

https://en.wikipedia.org/wiki/Extended_file_system

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.
extended as well b-tree

Answer 1

The data blocks of a very large file in the unix file system are allocated using an extension of indexed allocation or $\textsf{EXT2}$ file system. Hence, option (D) is the right answer.

Comments

Yes. But UFS and EXT2 are not the same. EXT2 is used in Linux but UFS existed even before.

(You can consider Unix as a base class and Linux and MAC its two derived classes)

http://en.wikipedia.org/wiki/Unix_File_System

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where is this question asked from?
is this in the syllabus?

where to find such questions at?

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CHAPTER 11- IMPLEMENTING FILE SYSTEM
TOPIC- ALLOCATION METHODS (INDEXED ALLOCATION)
page no- 477 galvin (8th edition)

Answer 2

ans D

The Unix file system uses an extension of indexed allocation. It uses direct blocks, single indirect blocks, double indirect blocks and triple indirect blocks

Answer 3

The correct answer is:

D. an extension of indexed allocation

The Unix File System (UFS) uses a variant of indexed allocation for managing data blocks. Specifically, it starts with:

• Indexed allocation via the inode structure (which acts like an index block).
• For large files, this is extended using:
  • Single indirect
  • Double indirect
  • Triple indirect block pointers

This approach extends the capacity of indexed allocation, allowing the system to manage very large files efficiently without needing contiguous or purely linked allocation methods.

Why not the others?

• A. Continuous allocation: Not used in Unix due to fragmentation and difficulty in growing files.
• B. Linked allocation: Not used because it is inefficient for random access.
• C. Indexed allocation: Only partially true; Unix goes beyond simple indexed allocation with multiple levels of indirection.

So, D is the most accurate.

Answer 4

1️⃣ Why not continuous allocation?

• Requires contiguous disk blocks

• Causes external fragmentation

• Not practical for very large or growing files

❌ Hence, not used.

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2️⃣ Why not linked allocation?

• Each block points to the next block

• Problems:

  • No direct access (must traverse block by block)

  • Pointer overhead

  • Poor performance for large files

❌ Inefficient for very large files.

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3️⃣ Why indexed allocation alone is not enough?

• Indexed allocation uses one index block to store addresses

• Index block size is limited

• For very large files, a single index block cannot store all block addresses

❌ Plain indexed allocation has a size limit.

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4️⃣ ✅ Why extension of indexed allocation is correct

Unix uses an extended indexed allocation scheme via inodes, which include:

• Direct pointers

• Single indirect pointer

• Double indirect pointer

• Triple indirect pointer

This allows:

• Efficient access for small files

• Massive scalability for very large files

✔ Supports extremely large file sizes
✔ Combines performance + scalability

Answer 5

The Unix File System (UFS) uses an inode structure to manage file allocations efficiently across varying file sizes.

For small files, the inode contains direct pointers that reference data blocks directly on the disk to optimize access speeds.

To handle very large files that exceed the storage capacity of these basic direct pointers, Unix implements a multi-level scheme consisting of single, double, and triple indirect pointers.

This multi-level indirect scheme functions as an extension of the basic indexed allocation method, dynamically increasing the number of addressable data blocks as the file grows.