NAME

file system − format of system volumes

SYNOPSIS

#include <sys/param.h>
#include <sys/fs.h>
#include <sys/inode.h>

DESCRIPTION

A file system is a discipline imposed by the UNIX operating system on a block-structured device, for the purpose of implementing the UNIX abstraction of a file.  Currently two types of file system are supported:  the Bell file system (formerly known as ‘the’ UNIX System V file system; see bellfs(4)), and the Extent file system (see efs(4)). Their common features are:

• A boot block at byte address 0. 

• A super block at byte address 512, containing such information as the number of blocks and the number of inodes contained in the file system. 

• A static division of the remaining file system space into inodes and data, determined when the file system is created. 

• One inode per file, containing its access permissions, the number of references (directory entries) to the file, its owner, its logical size, its times of last use and modification, and a list designating the blocks containing its data. 

• A free list designating blocks available for allocation to files. 

Their main differences are:

• Free list organization.  The Bell file system represents the free list as a chain of free blocks, each of which contains some number of block numbers, the first of which is the number of the next block in the chain. The Extent file system represents the free list as a bit map with a ‘1’ for each free block.

• Inode data block list.  The Bell file system represents the block list as a list of block numbers.  These block numbers appear in the inode itself and in multiply indirect blocks.  The Extent file system represents the block list as a list of extent descriptors (i.e., a logical offset, and a disk block number and a length).  The list appears in the inode itself if the number of extents is small; otherwise it appears in an array of indirect extents. 

• Inode position.  The Bell file system has a single inode region followed by a single data region.  The Extent file system has some number of cylinder groups, each of which consists of an inode region followed by a data region. 

SEE ALSO

bellfs(4), efs(4), fsck(1M), mkfs(1M), inode(4)

Version 3.6  —  December 20, 1987