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GO Classes DPP | OS | FILE SYSTEM

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An inode-based file system uses $4$ KB blocks and $4$-byte disk addresses. The inode contains $12$ direct block pointers, $1$ single indirect pointer, and $1$ double indirect pointer.

Which of the following statements is/are TRUE regarding this file system?

  1. THE MAXIMUM FILE SIZE SUPPORTED IS SLIGHTLY MORE THAN $4$ GB.
     
  2. THE MAXIMUM FILE SIZE SUPPORTED IS SLIGHTLY MORE THAN $1$ GB.
     
  3. ACCESSING A BYTE AT OFFSET $20$ MB REQUIRES AT MOST $3$ DISK BLOCK READS (INCLUDING THE INODE).
     
  4. A SINGLE INDIRECT BLOCK CAN ADDRESS UP TO $1024$ DATA BLOCKS.

1 Answer

1 1 vote

Block size $=4 \mathrm{~KB}$

Pointers per block $=\frac{4 \mathrm{~KB}}{4 \text { bytes }}=1024$ pointers

Direct: $12 \times 4 \mathrm{~KB}=48 \mathrm{~KB}$

Single Indirect: $1024 \times 4 \mathrm{~KB}=4 \mathrm{MB}$

Double Indirect: $1024 \times 1024 \times 4 \mathrm{~KB}=1024 \times 4 \mathrm{MB}=4 \mathrm{~GB}$

Total Size: $48 \mathrm{~KB}+4 \mathrm{MB}+4 \mathrm{~GB} \approx 4 \mathrm{~GB}$

(A) THE MAXIMUM FILE SIZE SUPPORTED IS SLIGHTLY MORE THAN $\mathbf{4}$ GB: TRUE. The total is $4 \mathrm{~GB}+4 \mathrm{MB}+48 \mathrm{~KB}$.

(B) THE MAXIMUM FILE SIZE SUPPORTED IS SLIGHTLY MORE THAN $\mathbf{1}$ GB: FALSE.

(C) ACCESSING A BYTE AT OFFSET $\mathbf{20}$ MB REQUIRES AT MOST 3 DISK BLOCK READS (INCLUDING THE INODE): FALSE.

  • $20$ MB is in the Double Indirect range (since Direct + Single Indirect only covers up to $\sim 4.04 \mathrm{MB}$ ).
     
  • Path: Inode → Double Indirect Block → First level Indirect Block → Data Block.
     
  • This requires $4$ reads $( 3$ disk blocks $+$ Inode$)$.
     

(D) A SINGLE INDIRECT BLOCK CAN ADDRESS UP TO $\mathbf{1024}$ DATA BLOCKS: TRUE.

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