Important short Questions and Answers: File Systems in OS

1 . What is a File?

A file is a named collection of related information that is recorded on secondary storage. A file contains either programs or data. A file has certain “structure” based on its type.

File attributes: Name, identifier, type, size, location, protection, time, date

File operations: creation, reading, writing, repositioning, deleting, truncating, appending, renaming

File types: executable, object, library, source code etc.

2. List the various File Attributes.

A file has certain other attributes, which vary from one operating system to another, but typically consist of these:  Name, identifier, type, location, size, protection, time, date and user identification.

3. What are the various File Operations?

The basic file operations are,

Creating a file

Writing a file

Reading a file

Repositioning within a file

Deleting a file

Truncating a file

4. What is the information associated with an Open File?

Several pieces of information are associated with an open file which may be: File pointer

File open count

Disk location of the file Access rights

5. What are the different Accessing Methods of a File?

The different types of accessing a file are:

Sequential access: Information in the file is accessed sequentially

Direct access: Information in the file can be accessed without any particular

order.

 Other access methods: Creating index for the file, indexed sequential access method (ISAM) etc.

6. What is Directory?

The device directory or simply known as directory records information- such as name, location, size, and type for all files on that particular partition. The directory can be viewed as a symbol table that translates file names into their directory entries.

7. What are the operations that can be performed on a Directory?

The operations that can be performed on a directory are,

Search for a file

Create a file

Delete a file

Rename a file

List directory

Traverse the file system

8. What are the most common schemes for defining the Logical Structure of aDirectory?

The most common schemes for defining the logical structure of a

SIngle directory Level Directory

Two level Directory

Tree Structured Directories

Acyclic-Graph Directories

Gentral Graph Directory

9. Define UFD and MFD.

In the two-level directory structure, each user has own user file directory (UFD). Each UFD has a similar structure, but lists only the files of a single user. When a job starts the system’s master file directory (MFD) is searched. The MFD is indexed by the user name or account number, and each entry points to the UFD for that user.

10. What is a Path Name?

A pathname is the path from the root through all subdirectories to a specified file. In a two-level directory structure a user name and a file name define a path name.

11. What is Access Control List (ACL)?

The most general scheme to implement identity-dependent access is to associate with each file and directory an access control unit.

12. Define Equal Allocation.

The way to split ‘m’ frames among ‘n’ processes is to give everyone an equal share, m/n frames. For instance, if there are 93 frames and 5 processes, each process will get 18 frames. The leftover 3 frames could be used as a free-frame buffer pool. This scheme is called equal allocation.

13. What is the cause of Thrashing? How does the system detect thrashing? Once it detects thrashing, what can the system do to eliminate this problem?

Thrashing is caused by under allocation of the minimum number of pages required by a process, forcing it to continuously page fault. The system can detect thrashing by evaluating the level of CPU utilization as compared to the level of multiprogramming. It can be eliminated by reducing the level of multiprogramming.

14. If the average page faults service time of 25 ms and a memory access time of 100ns.Calculate the effective access time.

Effective access time = (1-p)*ma + p*page fault time

= (1-p)*100+p*25000000

= 100-100p+25000000*p

= 100 + 24999900p

15.            What is Belady’s Anomaly?

For some page replacement algorithms, the page fault rate may increase as the number of allocated frames increases.

16. What are the different types of Access?

Different types of operations may be controlled in access type. These are,

Execute

Append

Delete

List

17. What are the types of Path Names?

Path names can be of two types.

 Absolute path  name: Begins  at  the root  and  follows a  path  down  to  the

specified file,  giving the directory names on the path.

 Relative path name: Defines a path from the current directory.

18. What is meant by Locality of Reference?

The locality model states that, as a process executes, it moves from locality to locality. Locality is of two types.

Spatial locality

Temporal locality

19. 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 organisation module

Basic ile system

I/O control

Device

20. What are the Structures used in File-System Implementation?

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

On disk structure include

Boot control block

Partition block

Directory structure used to organize the ile control block(FCB)

In memory structure include

In memory partition table

In memory directory structure

System wide open file table

Per process open table

21. What are the Functions of Virtual File System (VFS)?

It has two functions,

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.

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.

22. 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.

23. What are the Allocation Methods of a Disk Space?

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

Contiguous allocation

Linked allocation

Indexed allocation

24.     What are the advantages of Contiguous Allocation?

The advantages are,

Supports direct access

Supports sequential access

Number of disk seeks is minimal

25.     What are the drawbacks of Contiguous Allocation of Disk Space?

The disadvantages are,

Suffers from extrernal fragmentation

Suffers from internal fragmentation

Difficulty in finding space for a new file

File connot be extended

Size of the file is to be declared in advance

26. What are the advantages of Linked Allocation?

The advantages are,

No external fragmentation

Size of the file does not need to be declared

27. What are the disadvantages of Linked Allocation?

The disadvantages are,

Used only for sequential access of files.

Direct access is not supported

Memory space required for the pointers.

Reliability is compromised if the pointers are lost or damaged

28. What are the advantages of Indexed Allocation?

The advantages are,

No external fragmention problem

Solves the size declaration problems

Supports direct access

29. How can the index blocks be implemented in the Indexed Allocation Scheme?

The index block can be implemented as follows,

Linked scheme

Multilevel scheme

Combined scheme

30. Define Rotational Latency and Disk Bandwidth.

Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. The disk bandwidth is the total number of bytes transferred, divided by the time between the first request for service and the completion of the last transfer.

31. How free-space is managed using Bit Vector Implementation?

The free-space list is implemented as a bit map or bit vector. Each block is represented by 1 bit. If the block is free, the bit is 1; if the block is allocated, the bit is 0.

32. Define Buffering.

A buffer is a memory area that stores data while they are transferred between two devices or between a device and an application. Buffering is done for three reasons,

To Cope with a speed mismatch between the producer and consumer of a data stream

To adapt between devices that have dierent data transer size

To support copy semanties for application I/On