File allocation methods in os

This is a summary of my notes on operating systems file allocation methods. In the context of operating systems file allocation refers to managing files on disk such that disk space is effectively utilized and files are accessed quickly. Contiguous Allocation • • All blocks of the file are consecutive on disk • Deleting files leaves gaps • Compacting disk can be very slow • Simple and efficient indexing • Random access well supported • Difficult to grow: need to preallocate which wastes space • Easy to map from logical to physical address Let us assume we are going to read some position (pos) in a file. Assuming a block size of (1024) here is how to map from logical to physical address: Offset in block = pos % 1024 Linked Allocation • • Linked list of file blocks • Blocks can be scattered on disk • No limit on file size • File can grow easily • Random access well supported • Random access is not easy Using the same assumptions as before here is how to map from logical to physical address: Offset in block = pos % 1024 Linked Allocation Table in RAM Same as the previous file allocation method but the table resides in memory. It is faster but need to copy the table to disk at some point and keep both copies consistent. Index Block Allocation • • File block addresses are stored in an array which is stored in a disk block • Directory has a pointer to index block • Both random and sequential access are easy • File size limitation depends on array size for example if we have (256...

The main idea behind allocation is effective utilization of file space and fast access of the files. There are three types of allocation: • contiguous allocation • linked allocation • indexed allocation In addition to storing the actual file data on the disk drive, the file system also stores metadata about the files: the name of each file, when it was last edited, exactly where it is on the disk, and what parts of the disk are "free". Free areas are not currently in use by the file data or the metadata, and so available for storing new files. (The places where this metadata is stored are often called "inodes", "chunks", " To keep track of the free space, the file system maintains a free-space list which tracks all the disk blocks which are free. To create a file, the required space is reserved for the file and the corresponding space is removed from the free list linked to each other. Contiguous allocations [ | ] The contiguous allocation method requires each file to occupy a set of contiguous address on the disk. Disk addresses define a linear ordering on the disk. When a file has to be stored on a disk, system search for contiguous set of blocks as required by the file size i.e. system waits till it finds required number of memory blocks in sequence. When space is available system stores the file in the disk and makes an entry in the directory. The directory entry for a file with contigous allocation contains • Address of starting block • Length of the allocation block ...

Allocation Methods There are various methods which can be used to allocate disk space to the files. Selection of an appropriate allocation method will significantly affect the performance and efficiency of the system. Allocation method provides a way in which the disk will be utilized and the files will be accessed. There are following methods which can be used for allocation. • Contiguous Allocation. • Extents • Linked Allocation • Clustering • FAT • Indexed Allocation • Linked Indexed Allocation • Multilevel Indexed Allocation • Inode We will discuss three of the most used methods in detail.

A file is a collection of correlated information which is recorded on secondary or non-volatile storage like magnetic disks, optical disks, and tapes. It is a method of data collection that is used as a medium for giving input and receiving output from that program. In general, a file is a sequence of bits, bytes, or records whose meaning is defined by the file creator and user. Every File has a logical location where they are located for storage and retrieval. In this operating system tutorial, you will learn: • • • • • • • • • • • • Here are the main objectives of the file management system: • It provides I/O support for a variety of storage device types. • Minimizes the chances of lost or destroyed data • Helps OS to standardized I/O interface routines for user processes. • It provides I/O support for multiple users in a multiuser systems environment. Here, are important properties of a file system: • Files are stored on disk or other storage and do not disappear when a user logs off. • Files have names and are associated with access permission that permits controlled sharing. • Files could be arranged or more complex structures to reflect the relationship between them. A File Structure needs to be predefined format in such a way that an operating system understands. It has an exclusively defined structure, which is based on its type. Three types of files structure in OS: • A text file: It is a series of characters that is organized in lines. • An object file: It is a s...

This one is primarily intended to be used on portable media devices like flash drives, SDHC and SDXC cards, etc. exFAT officially supports portable media storage devices up to 512 TiB in size but theoretically could support drives as large as 64 ZiB, which is considerably larger than any media available as of this writing. Given their differences, if you place an encrypted file from an NTFS volume into a FAT-formatted space, the file loses its encryption status, meaning the file can be used like a normal, non-encrypted file. Decrypting a file in this way is only possible for the original user that encrypted the file, or any other user that has been granted permission by the original owner. Similar to encrypted files, since FAT doesn't support compression, a compressed file is automatically decompressed if it's copied out of an NTFS volume and onto a FAT volume. For example, if you copy a compressed file from an NTFS hard drive to a FAT floppy disk, the file will automatically decompress before it's saved to the floppy because the FAT file system on the destination media doesn't have the capability to store compressed files.

Overview A file system in an operating system decides the way the contents of its storage medium (secondary memory, etc) are stored and organized. Examples of filesystems are btrfs, xfs, zfs, ext series, ntfs, etc. These filesystems differ in aspects of implementation, use cases, etc. A filesystem is essential in the proper and efficient functioning of an operating system. Scope In this article we will discuss:- • What a filesystem is and its properties, common terms used in filesystems, and Allocation of space in a filesystem. • The structure of a file, its attributes and purposes, and the various mechanisms for accessing a file. • The various types of files, their names, and extensions. This article doesn't cover: • A case study of any filesystem in particular. • How filesystems are implemented. What is a Filesystem in OS? A file system in OS dictates how the contents of a storage medium are stored and organized. These storage media (such as secondary memory, external drives, etc) could be computer secondary memory, flash memory, etc. The contents are either files or directories. Most of the time, a storage device has a number of partitions. Each of these partitions is formatted with an empty filesystem for that device. A filesystem helps in separating the data on the storage into comparatively smaller and simpler segments. These chunks are files and directories. The filesystem also provides for storing data related to files, such as their name, extension, permissions, e...

A file system is a method an operating system uses to store, organize, and manage files and directories on a storage device. Some common types of file systems include: • FAT (File Allocation Table): An older file system used by older versions of Windows and other operating systems. • NTFS (New Technology File System): A modern file system used by Windows. It supports features such as file and folder permissions, compression, and encryption. • ext (Extended File System): A file system commonly used on Linux and Unix-based operating systems. • HFS (Hierarchical File System): A file system used by macOS. • APFS (Apple File System): A new file system introduced by Apple for their Macs and iOS devices. The advantages of using a file system include the: • Organization: A file system allows files to be organized into directories and subdirectories, making it easier to manage and locate files. • Data protection: File systems often include features such as file and folder permissions, backup and restore, and error detection and correction, to protect data from loss or corruption. • Improved performance: A well-designed file system can improve the performance of reading and writing data by organizing it efficiently on disk. Disadvantages of using a file system include: • Compatibility issues: Different file systems may not be compatible with each other, making it difficult to transfer data between different operating systems. • Disk space overhead: File systems may use some disk spa...

• File occupy contiguous blocks • Directory entry contains • Filename • Starting block • Length (total blocks) • Access possible • Sequential • Direct • Problems • Finding space for new file • External fragmentation • Determining space requirement of a file In the above example • File “sum” occupies blocks 2, 3 and 4. • File“jeep” occupies blocks 7, 8, 9, 10 and 11 • File “song” occupies blocks 15 and 16. Linked Allocation In linked file allocation method each file: • Directory structure contains • Pointer to the first and last block of file • Advantages • No external fragmentation • No issue with increase in file size • Disadvantages • Only sequential access • Reliability – loss of a pointer • Space required for pointers • Solution: make cluster of blocks • Problem: internal fragmentation In the above example • For file “car” the first data block is 8 • Block 8 contains data and a pointer to the next data block, which is block 3. • Block 3 points to the next data block i.e. 16 • Block 16 is the last block as pointed in the directory entry. Indexed Allocation For each file, the indexed file allocation method: • Clubs all the pointers into one block – index block • Directory entry contains • Filename • Index block number • Access • Direct • Issue • Size of index block • Sol: multilevel indexing In the above example • Block 8 is the index block which contains pointers to all the data blocks i.e., 10, 4, 12 and 19. • -1 entry in block 8 means it can contain entries to two oth...

A file is a collection of correlated information which is recorded on secondary or non-volatile storage like magnetic disks, optical disks, and tapes. It is a method of data collection that is used as a medium for giving input and receiving output from that program. In general, a file is a sequence of bits, bytes, or records whose meaning is defined by the file creator and user. Every File has a logical location where they are located for storage and retrieval. In this operating system tutorial, you will learn: • • • • • • • • • • • • Here are the main objectives of the file management system: • It provides I/O support for a variety of storage device types. • Minimizes the chances of lost or destroyed data • Helps OS to standardized I/O interface routines for user processes. • It provides I/O support for multiple users in a multiuser systems environment. Here, are important properties of a file system: • Files are stored on disk or other storage and do not disappear when a user logs off. • Files have names and are associated with access permission that permits controlled sharing. • Files could be arranged or more complex structures to reflect the relationship between them. A File Structure needs to be predefined format in such a way that an operating system understands. It has an exclusively defined structure, which is based on its type. Three types of files structure in OS: • A text file: It is a series of characters that is organized in lines. • An object file: It is a s...

Allocation Methods There are various methods which can be used to allocate disk space to the files. Selection of an appropriate allocation method will significantly affect the performance and efficiency of the system. Allocation method provides a way in which the disk will be utilized and the files will be accessed. There are following methods which can be used for allocation. • Contiguous Allocation. • Extents • Linked Allocation • Clustering • FAT • Indexed Allocation • Linked Indexed Allocation • Multilevel Indexed Allocation • Inode We will discuss three of the most used methods in detail.