Operating System

81.What are the various layers of a file system? 

The file system is composed of many different levels. Each level in the design uses the feature of the lower levels to create new features for use by higher levels. 

• Application programs 

• Logical file system 

• File-organization module 

• Basic file system 

• I/O control 

• Devices 

82.What are the structures used in file-system implementation? 

Several on-disk and in-memory structures are used to implement a file system 

a. On-disk structure include 

· Boot control block 

· Partition block 

· Directory structure used to organize the files 

· File control block (FCB) 

b. In-memory structure include 

· In-memory partition table 

· In-memory directory structure 

· System-wide open file table 

· Per-process open table 

83.What are the functions of virtual file system (VFS)? 

It has two functions 

a. It separates file-system-generic operations from their implementation defining a clean VFS interface. It allows transparent access to different types of file systems mounted locally. 

b. VFS is based on a file representation structure, called a vnode. It contains a numerical value for a network-wide unique file .The kernel maintains one vnode structure for each active file or directory. 

84.Define seek time and latency time. 

The time taken by the head to move to the appropriate cylinder or track is called seek time. Once the head is at right track, it must wait until the desired block rotates under the read-write head. This delay is latency time. 

85.What are the allocation methods of a disk space? 

Three major methods of allocating disk space which are widely in use are 

a. Contiguous allocation 

b. Linked allocation 

c. Indexed allocation 

86.What are the advantages of Contiguous allocation? 

The advantages are 

a. Supports direct access 

b. Supports sequential access 

c. Number of disk seeks is minimal. 

87.What are the drawbacks of contiguous allocation of disk space? 

The disadvantages are 

a. Suffers from external fragmentation 

b. Suffers from internal fragmentation 

c. Difficulty in finding space for a new file 

d. File cannot be extended 

e. Size of the file is to be declared in advance 

88.What are the advantages of Linked allocation? 

The advantages are 

a. No external fragmentation 

b. Size of the file does not need to be declared 

89.What are the disadvantages of linked allocation? 

The disadvantages are 

a. Used only for sequential access of files. 

b. Direct access is not supported 

c. Memory space required for the pointers. 

d. Reliability is compromised if the pointers are lost or damaged 

90.What are the advantages of Indexed allocation? 

The advantages are 

a. No external-fragmentation problem 

b. Solves the size-declaration problems. 

c. Supports direct access