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Journaling file system (JFS)
A file system in which the hard disk maintains data integrity in the event of a system crash or if the system is otherwise halted abnormally. The journaled file system (JFS) maintains a log, or journal, of what activity has taken place in the main data areas of the disk; if a crash occurs, any lost data can be recreated because updates to the metadata in directories and bit maps have been written to a serial log. The JFS not only returns the data to the pre-crash configuration but also recovers unsaved data and stores it in the location it would have been stored in if the system had not been unexpectedly interrupted.
File system
http://www.tech-analyser.com/2011/10/understanding-file-systemsntfs-fat.html
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device(s) which contain it. A file system organizes data in an efficient manner and is tuned to the specific characteristics of the device.
File systems specify conventions for naming files. These conventions include the maximum number of characters in a name, which characters can be used, and, in some systems, how long the file name suffix can be. A file system also includes a format for specifying the path to a file through the structure of directories.
Any computer file is stored on some kind of storage: hard disk, CD, DVD, flash memory and so on. These storages have specific, model-dependent capacity to store data. From the software point of view (including operating system) each storage is linear space to read or both read and write the digital information. Each byte of the information on a storage has its own specific offset from storage start (the address) and could be referenced by this address. You may imagine storage as grid with set of numbered cells (each cell – the single byte). Any file saved to storage takes a number of these cells.
Historically, computer storages like hard disk, CD, DVD or flash memory use pair of sector and in-sector offset to reference any byte of information on storage. The sector is the group of bytes (usually 512 bytes) that is minimum addressable unit of the physical storage. For example, byte 1030 on hard disk will be referenced as sector #3 and offset in sector 16 bytes ([sector]+[sector]+[16 bytes]). This schema is used to optimize storage addressing and use smaller number to reference any portion of information on the storage.
To omit second part of the address (the in-sector offset), files on storage are usually stored from the sector start and take all whole sectors (e.g.: 10 byte file takes a whole sector, 512 byte files also takes one sector, 514 byte file will take two whole sectors and so on).
Each file will be stored to ‘unused’ sectors and could be read then by known position and size. However, how do we know what sectors are used or unused? Where are file size and position stored? Where is file name? This answers give us the file system.
File system – is just a kind of structured data representation on a storage and set of metadata to describe the stored data. Unlike plain storage, file system could be located on disk partition – the isolated segment of storage. Usually it operates blocks, not sectors. The file system blocks are groups of sectors aimed at storage addressing optimization. Modern file systems generally use block sizes from 1 up to 128 sectors (512-65536 bytes). The files are usually stored from start of block and take entire blocks.
Many write/delete operations to file system could cause file system fragmentation: the files could not be stored as whole fragments anymore and are divided to fragments. Here is an example of “fragmentation”: imagine a storage entirely taken by files with size about 4 blocks (e.g. pictures collection). User wants to store a file that would take 8 blocks and therefore deletes the first and the last file. By doing this he releases 8 blocks, however the first segment is near to storage start, and the second near to storage end. In this case 8 block file will be split into two parts (4 blocks for each part) and will take free space ‘holes’. The information about both fragments which are parts of of a single file will be stored to file system.
Apart from user files, file system also stores its own parameters (as block size etc.), file descriptors (that include file size, file location, its fragments etc.), file names and directory hierarchy. It may store also security information, extended attributes and other parameters as well.
There are many requirements to storage performance, stability and other qualities of the file system. To best suit a specific purpose there have been developed many different types of file systems. So at present we can see plenty of file systems that are used on different types of computer systems and serve specific purposes.
Varous file systems :
NTFS (New Technology File System) – Windows
FAT (File Allocation Table) – Windows
ext2 (second extended file system) – Linux
ext3 (third extended file system) – Linux
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Techs it easy 