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AIX Version 4.3 System Management Guide: Operating System and Devices

File Systems Overview

A file system is a hierarchical structure (file tree) of files and directories. This type of structure resembles an inverted tree with the roots at the top and branches at the bottom. This file tree uses directories to organize data and programs into groups, allowing the management of several directories and files at one time.

Some tasks are performed more efficiently on a file system than on each directory within the file system. For example, you can back up, move, or secure an entire file system.

A file system resides on a single logical volume. The mkfs (make file system) command or the System Management Interface Tool (smit command) creates a file system on a logical volume. Every file and directory belongs to a file system within a logical volume.

To be accessible, a file system must be mounted onto a directory mount point. When multiple file systems are mounted, a directory structure is created that presents the image of a single file system. It is a hierarchical structure with a single root. This structure includes the base file systems and any file systems you create.

You can access both local and remote file systems using the mount command. This makes the file system available for read and write access from your system. Mounting or unmounting a file system usually requires system group membership. File systems can be mounted automatically, if they are defined in the /etc/filesystems file. You can unmount a local or remote file system with the umount command, unless a user or process is accessing that file system.

For information on the structure of the file system, see "Understanding the File Tree".

File System Types

Multiple file system types are supported. These include the following:

Journaled File System

The native file system type is called the journaled file system (JFS). It supports the entire set of file system semantics. This file system uses database journaling techniques to maintain its structural consistency. This prevents damage to the file system when the system is halted abnormally.

Each journaled file system resides on a separate logical volume. The operating system mounts journaled file systems during initialization. This multiple file system configuration is useful for system management functions such as backup, restore, and repair, because it isolates a part of the file tree so that you can work on it.

Network File System

The network file system (NFS) is a distributed file system that allows users to access files and directories located on remote computers and use those files and directories as if they were local. For example, users can use operating system commands to create, remove, read, write, and set file attributes for remote files and directories.

CD-ROM File System

The CD-ROM file system (CDRFS) is a file system type that allows you to access the contents of a CD-ROM through the normal file system interfaces. It is a read-only local file system implementation under the AIX logical file system (LFS) layer supporting the following volume and file structure formats:

The ISO 9660:1988(E) standard: The CDRFS supports ISO 9660 level 3 of interchange and level 1 of implementation.
The High Sierra Group Specification: Precedes the ISO 9660 and provides backward compatibility with previous CD-ROMs.
The Rock Ridge Group Protocol: Specifies extensions to the ISO 9660 that are fully compliant with the ISO 9660 standard, and that provide full POSIX file system semantics based on the System Use Sharing Protocol (SUSP) and the Rock Ridge Interchange Protocol (RRIP), enabling mount/access CD-ROM as with any other UNIX file system.
The CD-ROM eXtended Architecture File Format (in Mode 2 Form 1 sector format only)
                          The CD-ROM eXtended Architecture (XA) file format specifies extensions to the ISO 9660 that are used in CD-ROM-based multimedia applications for example, Photo CD.

For all volume and file structure formats, the following restrictions apply:

The CDRFS is dependent upon the underlying CD-ROM device driver to provide transparency of the physical sector format (CD-ROM Mode 1 and CD-ROM XA Mode 2 Form 1), and the multisession format of the disks (mapping the volume descriptor set from the volume recognition area of the last session).

File System Commands

There are a number of commands designed to operate on file systems, regardless of type. The /etc/filesystems file controls the list of file systems that the following commands can manipulate:

chfs Changes the characteristics of a file system.
crfs Adds a file system.
lsfs Displays the characteristics of a file system.
rmfs Removes a file system.
mount Makes a file system available for use.

Four commands operate on virtual file systems types. The /etc/vfs file contains the information on the file system types that the following commands manipulate:

chvfs Changes the characteristics of a file system type.
crvfs Adds a new file system type.
lsvfs Lists the characteristics of a file system type.
rmvfs Removes a file system type.

File System Management Tasks

A file system is a complete directory structure, including a root directory and any subdirectories and files beneath it. File systems are confined to a single logical volume. Some of the most important system management tasks have to do with file systems, specifically:

Following is a list of system management commands that are used regularly for working with file systems:

backup Performs a full or incremental backup of a file system.
dd Copies data directly from one device to another for making file system backups.
df Reports the amount of space used and free on a file system.
fsck Checks file systems and repairs inconsistencies.
mkfs Makes a file system of a specified size on a specified logical volume.
mount Attaches a file system to the systemwide naming structure so that files and directories in that file system can be accessed.
restore Restores files from a backup.
umount Removes a file system from the systemwide naming structure, making the files and directories in the file system inaccessible.

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