Operating System: File Management and File Systems.

File Management and File Systems

File Management and File Systems.

File systems

The collection of algorithms and data structures which perform the translation from logical file operations (system calls) to actual physical storage of information.

Objectives and requirements of a file system:

Objective of a file system include:

i) Provide data storage/ manipulation.

ii) Guarantee consistency of data and minimize errors.

iii) Optimize performance (system and user).

iv) Support variety of I/O device and provide standard system call interface.

v) Support multiple users.

Requirements to be fulfilled by a file system

i) User ability to create, delete, change files.

ii) Controlled access to other user’s files.

iii) User can restructure files as appropriate.

iv) Users should be able to back up and recover files.

v) Users should have access to files via a symbolic name.

File Access Methods

Advantages and disadvantages of sequential access.

Advantages :

i) Sequential files have simple organization principle.

ii) Sequential files have a range of applications.

iii) Sequential files are an efficient storage method if a large number of records are to be processed.

iv) Sequential files are inexpensive too use. Magnetic tape is not a costly medium.

Disadvantages:

i) Sequential master files must be completely processed and new ones created every time records are updated.

ii) Sequential files must usually be kept sorted.

iii) Sequential files are hard to keep current.

iv) Sequential files cannot be accessed directly.

Direct access & hash function

Direct access is based upon a disk model of a file. This method allows arbitrary bytes/records to be read or written.

In direct access file, records are stored on a direct access medium e.g. a disk, according to their addresses, which are often determined by hashing or hash function, of the address of a record is known, that record can be accessed directly without going through the records preceding it i.e. records can be accessed in any order. The address or location is determined, by hash function from the record itself. The hash function has two uses.

i) The function is used to determine where in he array to store record.

ii) The function id used as a method of accessing record.

List some advantages and disadvantages of direct access.

Advantages:

i) Direct access file eliminate the need for separate transaction files.

ii) Direct access files do not have go be kept sorted.

iii) More efficient processing is possible with direct access files.

iv) Retrieval of data sorted in direct access files is fast.

v) Several direct access files can be updated at the same time.

Disadvantages:

i) Backup files may need to be kept.

ii) Direct access files show greater potential for accidental data destruction and security breaches.

iii) Available storage space may be used less efficiently with direct access files than with sequential files.

iv) Complex hardware and software are needed to implement direct access files.

d) What are advantages of combined approach?

Ans: Advantages:

i) Indexed sequential files are well suited to both batched and individual transactions.

ii) With indexed sequential files, access to any particular record is faster than sequential files.

Security

The word security encompasses set of measures taken to guard against theft, attack, crime, and espionage or sabotage. Security implies the quality or state of being secure, that is, a relief from exposure to danger and acting so as to make safe against adverse contingencies.

Computer virus

Viruses are pieces of code that infect other programs and often perform harmful acts, such as deletion of files or corruption of the boot block. A virus is a program fragment that is attached to a legitimate program with the intention of infecting other programs. It differs from a worm only on that a virus piggybacks in an existing program, whereas a worm is a complete program in itself. Viruses and worms both attempt to spread themselves and both can do severe damage.

DAC

Discretionary access control (DAC): Policies are usually defines by the owner of data, who may pass access rights to other users. Usually, the creator of a file can specify the access rights of other users. This form of access control is common in file systems. It is vulnerable to the Trojan-horse attack, where intrudes pass themselves off as legitimate users.

MAC

Ans: Mandatory access control (MAC): Mandatory access restrictions are nor subject to user discretion and thus limit the damage that Trojan horse can cause. In this scheme, users are classified according to level of authority or clearance. Data are classified into security classes according to level of confidentiality, and strict rules are defined regarding which level of user clearance is required for accessing the data of a specific security class. For example, military documents are categorized as unclassified, confidential, secret, and top secret. The user is required to have clearance equal to or above that of systems on perhaps less obvious forms. For example, university administrators cannot pass the right to access grade records to students.

Security policy

Security policies specify what is desired in terms of protection and security. Securities encompass procedure and processes that specify:

i) How information can enter and exit the system

ii) Who is authorized to access what information and under what conditions.

iii) What are the permissible flows of information within the system?

Authentication

Threat monitoring

The system can check for suspicious patterns of activity jin an attempt to detect a security violation. For example, time sharing system which counts the number of incorrect passwords given when a user is trying to log in.

Different type of Biometrics techniques.

A Biometrics technique is based in the unique characteristics of each user. Some user characteristics can be relatively unobtrusively established by means of biometrics techniques. These are of two types:

i) Physiological characteristics, such as fingerprints, capillary patterns in the retina, hand geometry, and facial characteristics.

ii) Behavioral characteristics, such as signature analysis, voice pattern, and timing of keystrokes. On general, behavioral characteristics can very with a user’s state. For example signature dynamics and key strike patterns may very with a user’s stress level and fatigue

d) What do know about signature analysis?

The user signs his name with a special pen connected to the terminal, and the computer compares it to a known specimen stored on line. Even better is not to coma pare the signature, but compare the pen motions made while writing it.

The disadvantages of system generated passwords

Passwords have problems, but are extremely common, because they are easy to understand and use. The problems with passwords are related to difficulty of keeping a password secret. Passwords can be compromised by being guessed or accidentally written down where it can be read, or lost. Short passwords do not leave chough choices to prevent a password from being guessed by repeated trials. For example, a four digit password provides only 10,000 variations. In the average only 5.00 would need to be tried before the correct one would be guessed.