File Allocation methods in Operating Systems

Aug 20, 2024Download as pptx, pdf0 likes78 views

The presentation is about the allocation methods in Operating system containing detailed info like second level index blocks which inturn points to the disk blocks occupied by the file. This can be extended to 3 or more levels depending on the maximum file size. Combined Scheme: In this scheme, a special block called the Inode (information Node) contains all the information about the file such as the name, size, authority, etc and the remaining space of Inode is used to store the Disk Block addresses which contain the actual file as shown in the image below. The first few of these pointers in Inode point to the direct blocks i.e the pointers contain the addresses of the disk blocks that contain data of the file. The next few pointers point to indirect blocks. Indirect blocks may be single indirect, double indirect or triple indirect. Single Indirect block is the disk block that does not contain the file data but the disk address of the blocks that contain the file data. Similarly, double indirect blocks do not contain the file data but the disk address of the blocks that contain the address of the blocks containing the file data. inode

ALLOCATION METHODS
BY:M.SUMANASREE
MSCCOMPUTERSCIENCE1st
YEAR

File Allocation Methods
•The allocation methods define how the files are stored in the disk blocks. There
are three main disk space or file allocation methods
•.Contiguous Allocation
•Linked Allocation
•Indexed Allocation
•The main idea behind these methods is to provide:
•Efficient disk space utilization.
•Fast access to the file blocks.

1. Contiguous Allocation
•In this scheme, each file occupies a contiguous set of blocks on the disk. For
example, if a file requires n blocks and is given a block b as the starting
location, then the blocks assigned to the file will be:b, b+1, b+2,……b+n-1.This
means that given the starting block address and the length of the file (in terms
of blocks required), we can determine the blocks occupied by the file. The
directory entry for a file with contiguous allocation contains
•Address of starting block
•Length of the allocated portion.

Thefile ‘mail’in the following figure starts from the block 19 with length = 6
blocks. Therefore, it occupies19, 20, 21, 22, 23, 24blocks.

Advantages:
•Both the Sequential and Direct Accesses are supported by this. For direct
access, the address of the kth block of the file which starts at block b can
easily be obtained as (b+k).
•This is extremely fast since the number of seeks are minimal because of
contiguous allocation of file blocks.
•Disadvantages:
•This method suffers from both internal and external fragmentation. This
makes it inefficient in terms of memory utilization.
•Increasing file size is difficult because it depends on the availability of
contiguous memory at a particular instance.

2. Linked List Allocation
In this scheme, each file is a linked list of disk blocks
whichneed not becontiguous. The disk blocks can be
scattered anywhere on the disk. The directory entry
contains a pointer to the starting and the ending file
block. Each block contains a pointer to the next block
occupied by the file.The file ‘jeep’ in following image
shows how the blocks are randomly distributed. The last
block (25) contains -1 indicating a null pointer and does
not point to any other block.

Advantages:
• This is very flexible in terms of file size. File size can be increased easily since the
system does not have to look for a contiguous chunk of memory.
• This method does not suffer from external fragmentation. This makes it relatively
better in terms of memory utilization.
Disadvantages:
• Because the file blocks are distributed randomly on the disk, a large number of seeks
are needed to access every block individually. This makes linked allocation slower.
• It does not support random or direct access. We can not directly access the blocks of
a file. A block k of a file can be accessed by traversing k blocks sequentially (sequential
access ) from the starting block of the file via block pointers.
• Pointers required in the linked allocation incur some extra overhead.

3. Indexed Allocation
In this scheme, a special block known as
theIndex blockcontains the pointers to
all the blocks occupied by a file. Each file
has its own index block. The ith entry in
the index block contains the disk address
of the ith file block. The directory entry
contains the address of the index block
as shown in the image:

Advantages :
•This supports direct access to the blocks occupied by the file and therefore provides
fast access to the file blocks.
•It overcomes the problem of external fragmentation.
Disadvantages :
•The pointer overhead for indexed allocation is greater than linked allocation.
•For very small files, say files that expand only 2-3 blocks, the indexed allocation
would keep one entire block (index block) for the pointers which is inefficient in terms
of memory utilization. However, in linked allocation we lose the space of only 1 pointer
per block.

File Allocation methods in Operating Systems

The document discusses different methods for implementing directories and allocating disk space for files in a file system. There are two main directory implementation methods - linear list and hash table. Linear list allows simple programming but slow searches, while hash table can greatly decrease search time through mapping file names to list locations. For file allocation, contiguous allocation groups file blocks together but causes fragmentation, while linked and indexed allocation scatter blocks but require seeking between blocks. Indexed allocation supports direct access through a file index block but has higher overhead than linked allocation.

Allocation Methods-R.D.SivakumarSivakumar R D .

There are three major methods for storing files on disks: contiguous, linked, and indexed allocation. Contiguous allocation stores all blocks of a file together, providing fast sequential access but risks fragmentation. Linked allocation uses pointers between blocks, avoiding fragmentation but using more space for pointers. Indexed allocation combines all file indexes into common blocks, similar to file allocation tables used in DOS that store indexes in a separate table for faster random access. The optimal method depends on file size and access patterns.

Disk allocation methodsajeela mushtaq

This document discusses different disk allocation methods including contiguous, linked, and indexed allocation. Contiguous allocation stores files in contiguous disk blocks but can lead to external fragmentation. Linked allocation stores non-contiguous blocks linked through addresses in each block, avoiding fragmentation. Indexed allocation uses an index block to point to each block of a file, allowing for random access but requiring additional space for pointers.

Directory implementation and allocation methodssangrampatil81

FILE ALLOCATION METHODS.pptx ...........HiteshiArya

Operating System Unit 4(RTU Syllabus).pptxskultdedsec

Chapter 3Cahaya Penyayang

The document discusses file management in operating systems. It covers topics such as the basic functions of a file system, different file organization techniques like sequential, direct and indexed sequential, file structures, file allocation methods like contiguous, linked and indexed, free space management using free lists or bitmaps, file access control, and backup techniques. File systems are responsible for organizing files and managing access to data on storage devices in a way that facilitates navigation and protects data from corruption or loss.

Allocation methods (1).pptxssuser55cbdb

The document discusses different methods for allocating disk space and managing free space in file systems. It describes several allocation methods like contiguous allocation, linked allocation, clustering, FAT, indexed allocation, and inode allocation. It also covers approaches for tracking free disk blocks like using a bit vector or linked list of free blocks. The conclusion states that allocation methods impact file access performance and free space management techniques depend on the operating system and storage devices.

Allocation method - Operating System.pptxIndhu Periys

File allocation methods describe how an operating system stores file data in memory blocks on a hard drive. There are three main types: contiguous allocation stores a file in one continuous block; linked allocation connects file blocks in a linked list to avoid fragmentation; indexed allocation uses an index block to map file blocks and allows direct access. The methods aim to balance storage utilization and access time.

File Allocation Methods.pptBharathiLakshmiAAssi

This document summarizes different file system structures and allocation methods. It describes the logical layers of a file system from the application to the physical devices. It then explains three common allocation methods: contiguous allocation, linked allocation, and indexed allocation. Contiguous allocation stores files in contiguous disk blocks but can lead to fragmentation. Linked allocation uses pointers but is unreliable and wastes space. Indexed allocation uses an index block to point to other data blocks, reducing fragmentation and pointer space waste. It analyzes the performance of reading a block from each type of allocation.

File Concept.pptx fa s fasfasfasfsfsfasfasfasYuvarajY5

Allocation and free space managementrajshreemuthiah

1. File allocation methods include contiguous, linked, and indexed allocation. Contiguous allocation stores files in contiguous blocks but can lead to fragmentation. Linked allocation stores non-contiguous blocks through pointers but has overhead. Indexed allocation uses index blocks to point to data blocks. 2. File systems manage free space through data structures like bit vectors, linked lists, and space maps. Bit vectors require extra space but allow contiguous allocation. Linked lists have no wasted space but non-contiguous allocation. Space maps divide devices into metaslabs for efficient free space management. 3. Performance depends on allocation algorithms, metadata handling, buffer caching, and write policies. Techniques like read-ahead and free-behind optimize sequential access,

Chapter 3 part 1rohassanie

This document summarizes file management techniques including file organization, structure, and allocation methods. It discusses sequential, direct, and indexed sequential file organization and their characteristics. It also describes three methods of file allocation: contiguous, linked list using blocks, and linked list using an index. The linked list methods allow non-contiguous allocation by linking file extents through pointers.

File System ImplementationAbhishek Pachisia

Computers store files on disks in a file system that allows for structured access, reliability, and efficient use of space. A file system gives files names and attributes and stores them in a hierarchical directory structure. It implements techniques like hash tables to allow fast searching of file names and uses allocation methods like contiguous allocation to store file blocks efficiently and allow quick access.

Ch12_OS_Lecture 5.pdfAllinOne746595

This document discusses file allocation methods in operating systems. It describes three main allocation methods: contiguous allocation allocates a single contiguous set of blocks but can cause external fragmentation; chained allocation allocates blocks individually with pointers between them, avoiding fragmentation; and indexed allocation uses a block to store pointers to all other blocks for a file. The document compares the properties of these methods and how they are used to allocate space for files on secondary storage.

Secondary Storage ManagementPriyanshuGandhi3

The document discusses secondary storage management in operating systems. It addresses issues like file allocation, preallocation vs dynamic allocation, portion size, and different file allocation methods like contiguous, chained, and indexed allocation. It also covers free space management techniques like bit tables, chained free portions, indexing, and free block lists to track available blocks.

Allocation methods continuous method.47 myrajendra

The document discusses different allocation methods for managing disk space for files in an operating system. It describes contiguous, linked, and indexed allocation. Contiguous allocation allocates files in contiguous blocks, simplifying access but wasting space. Linked allocation uses pointers to non-contiguous blocks, avoiding fragmentation. Indexed allocation uses an index block to store block addresses, requiring multiple accesses to retrieve a block.

File sharingjanani thirupathi

This document discusses file sharing and secondary storage management in operating systems. It covers several topics: File sharing allows multiple users to access files, but access rights and simultaneous access must be managed. Access rights include permissions levels from none to deletion. Simultaneous access requires enforcing mutual exclusion to prevent conflicts. Secondary storage management involves allocating blocks to files from available disk space. File allocation methods include contiguous, chained, and indexed allocation. Contiguous allocates all blocks at once while chained uses pointers between non-contiguous blocks. Indexed addresses problems with the other methods. Free space is managed using techniques like bit tables to track used/free blocks, chained free portions, or a free block list maintained on disk

Filesharing 180214044607kirupasuchi1996

Report blocking ,management of files in secondry memory , static vs dynamic a...NoorMustafaSoomro

Three key topics were discussed: 1) Record blocking - the process of grouping related data records into blocks for storage. Fixed, variable-length spanned, and unspanned blocking methods were described. 2) File management in secondary memory - files have attributes like name, size, permissions. Common file operations are create, open, read, write. Directory structures and access paths organize files. 3) Memory allocation - static allocation assigns memory at compile time while dynamic allocation occurs at runtime using functions like malloc(), free(), calloc(), realloc(). Contiguous, linked lists and indexing are approaches to storing files and managing free space.

File implementationMohd Arif

The document discusses file system implementation and storage management. It covers topics like file system structure, file control blocks, directory implementation methods, file allocation methods including contiguous, linked, and indexed allocation, and free space management techniques like bit vectors, linked lists, and address counting. The key aspects of different file allocation and free space tracking methods are compared.

Fileskirtidhamija16

Files are sequences of data organized in a structure defined by their creator. They have attributes like a name, size, and permissions. There are different file structures - text files are sequences of characters organized into lines, while object files contain machine-readable blocks. Files are stored in a directory and accessed using methods like sequential, direct, or indexed sequential access. The operating system allocates disk space to files using allocation methods like contiguous, linked, or indexed allocation and provides protection through access controls and permissions.

Files Allocation.pptChinmaya M. N

An inode is a data structure that contains metadata about a file, including its location on disk. It is associated with a file through its inode number. Directories contain lists of filenames and pointers to inodes. UNIX uses a multilevel indexing system for file allocation, where inodes contain addresses of data blocks and index blocks that point to additional blocks, allowing files to grow larger than a single block. This supports both sequential and direct access.

SSMRamasubbu .P

Secondary storage management involves tracking portions (groups of contiguous blocks) assigned to files using a File Allocation Table (FAT). There are two main approaches to file allocation - preallocation which requires estimating maximum file sizes up front, and dynamic allocation which allocates portions as needed. Common file allocation methods include contiguous, chained, and indexed allocation, each with advantages and disadvantages for sequential vs random access performance and fragmentation. Free space on disks is managed using techniques like bit tables, chained free portions lists, or indexes to track available blocks.

oslectureset7.pdfWaltonJrSmithTZ

This document provides an overview of file systems and their implementation in operating systems. It discusses key concepts like file structure, attributes, operations, directory structures including tree and graph organizations, allocation methods like contiguous, linked and indexed, access methods, protection, and efficiency techniques. The document also describes UNIX inodes which combine different allocation schemes and provide metadata for files.

Dude.pptxDushyantChauhan40

8 File Management system project .pptxanitabricks49

YSPH VMOC Special Report - Measles Outbreak Southwest US 5-25-2025.pptxYale School of Public Health - The Virtual Medical Operations Center (VMOC)

CURRENT CASE COUNT: 880 • Texas: 729 (+5) (56% of cases are in Gaines County) • New Mexico: 78 (+4) (83% of cases are from Lea County) • Oklahoma: 17 • Kansas: 56 (38.89% of the cases are from Gray County) HOSPITALIZATIONS: 103 • Texas: 94 - This accounts for 13% of all cases in the State. • New Mexico: 7 – This accounts for 9.47% of all cases in New Mexico. • Kansas: 2 - This accounts for 3.7% of all cases in Kansas. DEATHS: 3 • Texas: 2 – This is 0.28% of all cases • New Mexico: 1 – This is 1.35% of all cases US NATIONAL CASE COUNT: 1,076 (confirmed and suspected) INTERNATIONAL SPREAD • Mexico: 1,753 (+198) 4 fatalities ‒ Chihuahua, Mexico: 1,657 (+167) cases, 3 fatalities, 9 hospitalizations • Canada: 2518 (+239) (Includes Ontario’s outbreak, which began November 2024) ‒ Ontario, Canada: 1,795 (+173) 129 (+10) hospitalizations ‒ Alberta, Canada: 560 (+55) Things to keep an eye on: Mexico: Three children have died this month (all linked to the Chihuahua outbreak): An 11-month-old and a 7-year-old with underlying conditions A 1-year-old in Sonora whose family is from Chihuahua Canada: Ontario now reports more cases than the entire U.S. Alberta’s case count continues to climb rapidly and is quickly closing in on 600 cases. Emerging transmission chains in Manitoba and Saskatchewan underscore the need for vigilant monitoring of under-immunized communities and potential cross-provincial spread. United States: North Dakota: Grand Forks County has confirmed its first cases (2), linked to international travel. The state total is 21 since May 2 (including 4 in Cass County and 2 in Williams County), with one hospitalization reported. OUTLOOK: With the spring–summer travel season peaking between Memorial Day and Labor Day, both domestic and international travel may fuel additional importations and spread. Although measles transmission is not strictly seasonal, crowded travel settings increase the risk for under-immunized individuals.

Unit 2 DNS Spoofing in a BadUSB Attack.pdfChatanBawankar

More Related Content

Similar to File Allocation methods in Operating Systems (20)

Allocation methods (1).pptxssuser55cbdb

Allocation method - Operating System.pptxIndhu Periys

File Allocation Methods.pptBharathiLakshmiAAssi

File Concept.pptx fa s fasfasfasfsfsfasfasfasYuvarajY5

Allocation and free space managementrajshreemuthiah

Chapter 3 part 1rohassanie

File System ImplementationAbhishek Pachisia

Ch12_OS_Lecture 5.pdfAllinOne746595

Secondary Storage ManagementPriyanshuGandhi3

Allocation methods continuous method.47 myrajendra

File sharingjanani thirupathi

Filesharing 180214044607kirupasuchi1996

Report blocking ,management of files in secondry memory , static vs dynamic a...NoorMustafaSoomro

File implementationMohd Arif

Fileskirtidhamija16

Files Allocation.pptChinmaya M. N

SSMRamasubbu .P

oslectureset7.pdfWaltonJrSmithTZ

Dude.pptxDushyantChauhan40

8 File Management system project .pptxanitabricks49

Allocation methods (1).pptxssuser55cbdb

Allocation method - Operating System.pptxIndhu Periys

File Allocation Methods.pptBharathiLakshmiAAssi

File Concept.pptx fa s fasfasfasfsfsfasfasfasYuvarajY5

Allocation and free space managementrajshreemuthiah

Chapter 3 part 1rohassanie

File System ImplementationAbhishek Pachisia

Ch12_OS_Lecture 5.pdfAllinOne746595

Secondary Storage ManagementPriyanshuGandhi3

Allocation methods continuous method.47 myrajendra

File sharingjanani thirupathi

Filesharing 180214044607kirupasuchi1996

Report blocking ,management of files in secondry memory , static vs dynamic a...NoorMustafaSoomro

File implementationMohd Arif

Fileskirtidhamija16

Files Allocation.pptChinmaya M. N

SSMRamasubbu .P

oslectureset7.pdfWaltonJrSmithTZ

Dude.pptxDushyantChauhan40

8 File Management system project .pptxanitabricks49

Recently uploaded (20)

YSPH VMOC Special Report - Measles Outbreak Southwest US 5-25-2025.pptxYale School of Public Health - The Virtual Medical Operations Center (VMOC)

Unit 2 DNS Spoofing in a BadUSB Attack.pdfChatanBawankar

Unit Kali NetHunter is the official Kali Linux penetration testing platform f...ChatanBawankar

"Orthoptera: Grasshoppers, Crickets, and Katydids pptxArshad Shaikh

How to Use Owl Slots in Odoo 17 - Odoo SlidesCeline George

Unit 1 Kali NetHunter is the official Kali Linux penetration testing platform...ChatanBawankar

STUDENT LOAN TRUST FUND DEFAULTERS GHANAKweku Zurek

Understanding-the-Weather.pdf/7th class/social/ 2nd chapter/Samyans Academy n...Sandeep Swamy

Drug Metabolism advanced medicinal chemistry.pptxpharmaworld

This document describes about structural metabolism relationship and drug designing and toxicity of drugs in " DRUG METABOLISM" In Drug Metabolism is the process of converting a drug into product or inert substances after or before reaching at the site of action. Metabolism plays an important role in elimination of drugs and foreign substance from the body. The metabolism of any drug is generally characterised by two phases of reaction 1.Metabolic transformation ( biotransformation ) and 2.Conjugation The Principal site of drug metabolism is the liver, but the kidney, lungs, and GIT also are important metabolic sites. The enzymatic bio transformations of drugs is known as Drug Metabolism. Because many drugs have structures similar to those of endogenous compounds , drugs may get metabolised by specific enzymes for the related natural substrates as well as by non-specific enzymes. Reaction type of Phase-I: 1.Oxidation 2.Reduction 3.Hydrolysis Most drugs are metabolised ,atleast to some extent , by both phases of metabolism. Example: Metabolism of Aspirin Acetyl Salicylic acid undergoes hydrolysis to salicylic acid ( metabolic transformation ), which is then conjugated with glycine to form Salicyluric acid ( Conjugation ). In Phase-II the metabolites formed in Phase-II are converted to more polar and water soluble product by attaching polar and ionisable moiety such as 1.Glucuronic acid 2.Glycine 3.Glutamine 4.Glutathione conjugation 5.Acetylation 6.Methylation 7.Sulfate conjugation 8.Nucleoside and Nucleotide formation 9.Amino Acid Conjugation 10.Fatty Acid and Cholesterol Conjugation Drug design is the process of creating new drugs by using knowledge of a biological target. Drug design considers metabolism to optimize pharmaco kinects ( ADME: Absorption , Distribution , Metabolism , Excretion ) Cytochrome CYP450 enzyme in Drug Metabolism is vital in drug design to enhance efficacy , reduce toxicity and improve bioavailability. Cytochrome P450 enzymes (CYPs) are a superfamily of heme -containing proteins found from bacteria to human. Cytochrome P-450 activity in various organs like Liver,Lung ,Kidney , Intestine,Placenta Adrenal and Skin and they shows the relative activity with repect to their organs in the process of drug metabolism. Most important CYP450 enzymes are CYP1A2 , CYP2C9 , CYP2E1 ,etc... Toxic Effects of Drug Metabolism Toxicity: Accumulation of Excess of medications in the Blood Stream. Ariens (1948) and Mitchell and Horning (1984) deal with this topic. Some examples of Metabolism-Linked Toxicity are 1.Acetaminophen (paracetmol) 2.Isoniazid ( TB drug) 3.Chloroform 4.Dapsone 5.Diazepam 6.Salicylate 7.Halothane (Anesthetic) 8.Tamoxifen (Breast Cancer drug ) 9.Clozapine(Antipsychotic) These drugs are differentiates with the TOXIC METABOLITE , TOXICITY OF METABOLITE. References for this topic also mentioned at the end.

KNN,Weighted KNN,Nearest Centroid Classifier,Locally Weighted RegressionGlobal Academy of Technology

Odoo 18 Point of Sale PWA - Odoo SlidesCeline George

Optical and non optical devices used in low visionAligarh Muslim University, Aligarh, Uttar Pradesh, India

Multicultural approach in education - B.Edprathimagowda443

Regression Analysis-Machine Learning -Different TypesGlobal Academy of Technology

What are the Features & Functions of Odoo 18 SMS MarketingCeline George

Unit 4 Reverse Engineering Tools Functionalities & Use-Cases.pdfChatanBawankar

LDMMIA About me 2025 Edition 3 College VolumeLDM & Mia eStudios

Updated About Me. Used for former college assignments. Make sure to catch our weekly updates. Updates are done Thursday to Fridays or its a holiday/event weekend. Thanks again, Readers, Guest Students, and Loyalz/teams. This profile is older. I started at the beginning of my HQ journey online. It was recommended by AI. AI was very selective but fits my ecourse style. I am media flexible depending on the course platform. More information below. AI Overview: “LDMMIA Reiki Yoga refers to a specific program of free online workshops focused on integrating Reiki energy healing techniques with yoga practices. These workshops are led by Leslie M. Moore, also known as LDMMIA, and are designed for all levels, from beginners to those seeking to review their practice. The sessions explore various themes like "Matrix," "Alice in Wonderland," and "Goddess," focusing on self-discovery, inner healing, and shifting personal realities.”

EVALUATION AND MANAGEMENT OF OPEN FRACTUREBipulBorthakur

[2025] Qualtric XM-EX-EXPERT Study Plan | Practice Questions + Exam DetailsJenny408767

The Ellipsis Manual Analysis And Engineering Of Human Behavior Chase Hughespekokmupei

YSPH VMOC Special Report - Measles Outbreak Southwest US 5-25-2025.pptxYale School of Public Health - The Virtual Medical Operations Center (VMOC)

Unit 2 DNS Spoofing in a BadUSB Attack.pdfChatanBawankar

Unit Kali NetHunter is the official Kali Linux penetration testing platform f...ChatanBawankar

"Orthoptera: Grasshoppers, Crickets, and Katydids pptxArshad Shaikh

How to Use Owl Slots in Odoo 17 - Odoo SlidesCeline George

Unit 1 Kali NetHunter is the official Kali Linux penetration testing platform...ChatanBawankar

STUDENT LOAN TRUST FUND DEFAULTERS GHANAKweku Zurek

Understanding-the-Weather.pdf/7th class/social/ 2nd chapter/Samyans Academy n...Sandeep Swamy

Drug Metabolism advanced medicinal chemistry.pptxpharmaworld

KNN,Weighted KNN,Nearest Centroid Classifier,Locally Weighted RegressionGlobal Academy of Technology

Odoo 18 Point of Sale PWA - Odoo SlidesCeline George

Optical and non optical devices used in low visionAligarh Muslim University, Aligarh, Uttar Pradesh, India

Multicultural approach in education - B.Edprathimagowda443

Regression Analysis-Machine Learning -Different TypesGlobal Academy of Technology

What are the Features & Functions of Odoo 18 SMS MarketingCeline George

Unit 4 Reverse Engineering Tools Functionalities & Use-Cases.pdfChatanBawankar

LDMMIA About me 2025 Edition 3 College VolumeLDM & Mia eStudios

EVALUATION AND MANAGEMENT OF OPEN FRACTUREBipulBorthakur

[2025] Qualtric XM-EX-EXPERT Study Plan | Practice Questions + Exam DetailsJenny408767

The Ellipsis Manual Analysis And Engineering Of Human Behavior Chase Hughespekokmupei

File Allocation methods in Operating Systems

1. ALLOCATION METHODS BY:M.SUMANASREE MSCCOMPUTERSCIENCE1st YEAR

2. File Allocation Methods •The allocation methods define how the files are stored in the disk blocks. There are three main disk space or file allocation methods •.Contiguous Allocation •Linked Allocation •Indexed Allocation •The main idea behind these methods is to provide: •Efficient disk space utilization. •Fast access to the file blocks.

3. 1. Contiguous Allocation •In this scheme, each file occupies a contiguous set of blocks on the disk. For example, if a file requires n blocks and is given a block b as the starting location, then the blocks assigned to the file will be:b, b+1, b+2,……b+n-1.This means that given the starting block address and the length of the file (in terms of blocks required), we can determine the blocks occupied by the file. The directory entry for a file with contiguous allocation contains •Address of starting block •Length of the allocated portion.

4. Thefile ‘mail’in the following figure starts from the block 19 with length = 6 blocks. Therefore, it occupies19, 20, 21, 22, 23, 24blocks.

5. Advantages: •Both the Sequential and Direct Accesses are supported by this. For direct access, the address of the kth block of the file which starts at block b can easily be obtained as (b+k). •This is extremely fast since the number of seeks are minimal because of contiguous allocation of file blocks. •Disadvantages: •This method suffers from both internal and external fragmentation. This makes it inefficient in terms of memory utilization. •Increasing file size is difficult because it depends on the availability of contiguous memory at a particular instance.

6. 2. Linked List Allocation In this scheme, each file is a linked list of disk blocks whichneed not becontiguous. The disk blocks can be scattered anywhere on the disk. The directory entry contains a pointer to the starting and the ending file block. Each block contains a pointer to the next block occupied by the file.The file ‘jeep’ in following image shows how the blocks are randomly distributed. The last block (25) contains -1 indicating a null pointer and does not point to any other block.

7. Advantages: • This is very flexible in terms of file size. File size can be increased easily since the system does not have to look for a contiguous chunk of memory. • This method does not suffer from external fragmentation. This makes it relatively better in terms of memory utilization. Disadvantages: • Because the file blocks are distributed randomly on the disk, a large number of seeks are needed to access every block individually. This makes linked allocation slower. • It does not support random or direct access. We can not directly access the blocks of a file. A block k of a file can be accessed by traversing k blocks sequentially (sequential access ) from the starting block of the file via block pointers. • Pointers required in the linked allocation incur some extra overhead.

8. 3. Indexed Allocation In this scheme, a special block known as theIndex blockcontains the pointers to all the blocks occupied by a file. Each file has its own index block. The ith entry in the index block contains the disk address of the ith file block. The directory entry contains the address of the index block as shown in the image:

9. Advantages : •This supports direct access to the blocks occupied by the file and therefore provides fast access to the file blocks. •It overcomes the problem of external fragmentation. Disadvantages : •The pointer overhead for indexed allocation is greater than linked allocation. •For very small files, say files that expand only 2-3 blocks, the indexed allocation would keep one entire block (index block) for the pointers which is inefficient in terms of memory utilization. However, in linked allocation we lose the space of only 1 pointer per block.

File Allocation methods in Operating Systems

Recommended

More Related Content

Similar to File Allocation methods in Operating Systems (20)

Recently uploaded (20)

File Allocation methods in Operating Systems