Automounter
Encyclopedia
An automounter is any program or software facility which automatically mount
s filesystems in response to access operations by user programs. An automounter system utility (daemon
under Unix
), when notified of file and directory access attempts under selectively monitored subdirectory trees, dynamically and transparently makes local or remote devices accessible.
The automounter has the purpose of conserving local system resources and of reducing the coupling between systems which share filesystems with a number of servers. For example, a large to mid-sized organization might have hundreds of file server
s and thousands of workstation
s or other nodes
accessing files from any number of those servers at any time. Usually, only a relatively small number of remote filesystems (exports) will be active on any given node at any given time. Deferring the mounting of such a filesystem until a process actually needs to access it reduces the need to track such mounts, increasing reliability, flexibility and performance.
Frequently, one or more fileservers will become inaccessible (down for maintenance, on a remote and temporarily disconnected network, or accessed via a congested link). Administrators also often find it necessary to relocate data from one file server to another - to resolve capacity issues and balance the load. Having data mount-points automated makes it easier to reconfigure client systems in such cases. In addition, end-users should only have the ability to mount some removable media
devices - such as floppies, CD-ROMs, and USB keys - when the device is attached to the system.
These factors combine to pose challenges to older "static" management methods of filesystem mount tables (the
One can also use automounters to define multiple repositories for read-only data; client systems can automatically choose which repository to mount based on availability, file-server load, or proximity on the network.
of various users. All workstations and other nodes internal to such organizations (typically all those behind a common firewall
separating them from the Internet
) will be configured with automounter services so that any user logging into any node implicitly triggers access to his or her own home directory which, consequently, is mounted at a common mountpoint, such as
where
Commonly, organizations will provide repositories or "depots" of such software, ready for installation as required. These also may include full copies of the system images from which machines have their operating systems initially installed, or available for repair of any system files that may get corrupted during a machine's lifecycle.
Some software may require quite substantial storage space or might be undergoing rapid (perhaps internal) development. In those cases the software may be installed on, and configured to be run directly from, the file servers.
For situations like this, automounter utilities generally support some means of "mapping" or "interpolating" variable data into the mount arguments.
For example, an organization with a mixture of Linux
and Solaris systems might arrange to host their software package
repositories for each on a common file-server using export names like
or CentOS
(Linux) user would find RPMs for their particular OS version thereunder. Moreover, a Solaris user on a SPARC
workstation would have his
would transparently find his
or Python
) can be installed and/or run on any supported platform without porting, recompilation or re-packaging of any sort. A systems administrator might conceivably locate such software in a
In some cases, organizations may also use regional or location-based variable/dynamic mappings — so that users in one building or site are directed to a closer file server which hosts replications of the resources that are hosted at other locations.
In all of these cases, automounter utilities allow the users to access files and directories without regard for the actual physical location. Using an automounter, the users and systems administrators can usually access files where they are "supposed to be" and find that they appear to be there.
: SunOS
4.0 made automounting available in 1988. Sun Microsystems eventually licensed this implementation to other commercial UNIX distributions. Solaris 2.0, first released in 1992, implemented its automounter with a pseudofilesystem called
systems have adopted that implementation of the automounter - including AIX, HP-UX
, and Mac OS X 10.5 and later.
Linux
has an independent implementation of an autofs-based automounter; version 5 of that automounter generally operates compatibly with the Solaris automounter.
In December 1989 Jan-Simon Pendry released amd, an automounter "based in spirit" on the SunOS automount program. amd has also become known as the Berkeley Automounter
.
Some operating systems also support automatic mounting of external drives (such as disk drives or flash drives that use FireWire or USB connections) and removable media (such as CDs and DVD
s). This technology differs from the automounting described here; it involves mounting local media when the user attaches them to or inserts them into the system, rather than mounting directories from remote file servers when a reference is made to them. Linux currently (as of Linux 2.6) uses the user-space program udev
for this form of automounting. Some automounting functions have been implemented in the separate program HAL
, but are being merged into udev. OpenBSD has hotplugd(8) which triggers special scripts on attach or detach of removable devices, so that user can easily add mounting of removable drives. In Mac OS X
Mount (computing)
Mounting takes place before a computer can use any kind of storage device . The user or their operating system must make it accessible through the computer's file system. A user can access only files on mounted media.- Mount point :A mount point is a physical location in the partition used as a...
s filesystems in response to access operations by user programs. An automounter system utility (daemon
Daemon (computer software)
In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...
under Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
), when notified of file and directory access attempts under selectively monitored subdirectory trees, dynamically and transparently makes local or remote devices accessible.
The automounter has the purpose of conserving local system resources and of reducing the coupling between systems which share filesystems with a number of servers. For example, a large to mid-sized organization might have hundreds of file server
File server
In computing, a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access, i.e. shared storage of computer files that can be accessed by the workstations that are attached to the computer network...
s and thousands of workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
s or other nodes
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
accessing files from any number of those servers at any time. Usually, only a relatively small number of remote filesystems (exports) will be active on any given node at any given time. Deferring the mounting of such a filesystem until a process actually needs to access it reduces the need to track such mounts, increasing reliability, flexibility and performance.
Frequently, one or more fileservers will become inaccessible (down for maintenance, on a remote and temporarily disconnected network, or accessed via a congested link). Administrators also often find it necessary to relocate data from one file server to another - to resolve capacity issues and balance the load. Having data mount-points automated makes it easier to reconfigure client systems in such cases. In addition, end-users should only have the ability to mount some removable media
Removable media
In computer storage, removable media refers to storage media which is designed to be removed from the computer without powering the computer off.Some types of removable media are designed to be read by removable readers and drives...
devices - such as floppies, CD-ROMs, and USB keys - when the device is attached to the system.
These factors combine to pose challenges to older "static" management methods of filesystem mount tables (the
fstabFstabThe fstab file is a system configuration file commonly found on Unix systems. The fstab file typically lists all available disks and disk partitions, and indicates how they are to be initialized or otherwise integrated into the overall system's file system...
files on Unix systems). Automounter utilities address these challenges and allow sysadmins to consolidate and centralize the associations of mountpoints (directory names) to the exports. When done properly, users can transparently access files and directories as if all of their workstations and other nodes attach to a single enterprise-wide filesystem.One can also use automounters to define multiple repositories for read-only data; client systems can automatically choose which repository to mount based on availability, file-server load, or proximity on the network.
Home directories
Many establishments will have a number of file servers which host the home directoriesHome directory
A Home directory is a file system directory on a multi-user operating system containing files for a given user of the system. The specifics of the home directory is defined by the operating system involved; for example, Windows systems between 2000 and 2003 keep home directories in a folder...
of various users. All workstations and other nodes internal to such organizations (typically all those behind a common firewall
Firewall (computing)
A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass....
separating them from the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
) will be configured with automounter services so that any user logging into any node implicitly triggers access to his or her own home directory which, consequently, is mounted at a common mountpoint, such as
/home/user. This allows users to access their own files from anywhere in the enterprise, which is extremely useful in Unix environments, where users may frequently invoke commands on many remote systems via various job-dispatching commands such as sshSecure ShellSecure Shell is a network protocol for secure data communication, remote shell services or command execution and other secure network services between two networked computers that it connects via a secure channel over an insecure network: a server and a client...
, telnetTELNETTelnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility using a virtual terminal connection...
, rshRSHRSH may mean:*The NYSE stock symbol for RadioShack*Remote shell, a UNIX command-line utility for remotely executing commands*Restricted shell, a restricted shell environment for Unix*Robert Stephenson and Hawthorns, a locomotive manufacturer...
or rloginRloginrlogin is a software utility for Unix-like computer operating systems that allows users to log in on another host via a network, communicating via TCP port 513.It was first distributed as part of the 4.2BSD release....
, or via the X11 or VNC protocols./net
A very common default automounter local path is of the form/net/hostname/nfspathwhere
hostname is the host name of the remote machine and nfspath is the path that is exported over NFS on the remote machine. This notation generally frees the system manager from having to manage each exported path explicitly via a central automounter map.Software shares and repositories
In some computing environments, user workstations and computing nodes do not host installations of the full range of software that users might want to access. Systems may be "imaged" with a minimal or typical cross-section of the most commonly used software. Also, in some environments, users might require specialized or occasional access to older versions of software (for instance, developers may need to perform bug fixes and regression testing, or some users may need access to archived data using outdated tools).Commonly, organizations will provide repositories or "depots" of such software, ready for installation as required. These also may include full copies of the system images from which machines have their operating systems initially installed, or available for repair of any system files that may get corrupted during a machine's lifecycle.
Some software may require quite substantial storage space or might be undergoing rapid (perhaps internal) development. In those cases the software may be installed on, and configured to be run directly from, the file servers.
Dynamically variant automounts
In the simplest case, a fileserver houses data and perhaps scripts which can be accessed by any system in an environment. However, certain types of files (executable binaries and shared libraries, in particular) can only be used by specific types of hardware or specific versions of specific operating systems.For situations like this, automounter utilities generally support some means of "mapping" or "interpolating" variable data into the mount arguments.
For example, an organization with a mixture of Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
and Solaris systems might arrange to host their software package
Software package (installation)
In package management systems, which are commonly used with Linux-based operating systems, a package is a specific piece of software which the system can install and uninstall....
repositories for each on a common file-server using export names like
depot:/export/linux and depot:/export/solaris respectively. Thereunder they might have directories for each of the OS versions that they support. Using the dynamic variation features in their automounter, they might then configure all their systems so that any administrator on any machine in their enterprise could access available software updates under /software/updates. A user on a Solaris system would find the Solaris compiled packages under /software, while a Red HatRed Hat
Red Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....
or CentOS
CentOS
CentOS is a free operating system based on Red Hat Enterprise Linux . It exists to provide a free enterprise class computing platform and strives to maintain 100% binary compatibility with its upstream distribution...
(Linux) user would find RPMs for their particular OS version thereunder. Moreover, a Solaris user on a SPARC
SPARC
SPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
workstation would have his
/software/updates mapped to an appropriate export for that system's architecture, while a Solaris user on an x86 PCPersonal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
would transparently find his
/software/updates directory containing packages suited to his system. Some software (written in scripting languages such as PerlPerl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
or Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
) can be installed and/or run on any supported platform without porting, recompilation or re-packaging of any sort. A systems administrator might conceivably locate such software in a
/software/common export.In some cases, organizations may also use regional or location-based variable/dynamic mappings — so that users in one building or site are directed to a closer file server which hosts replications of the resources that are hosted at other locations.
In all of these cases, automounter utilities allow the users to access files and directories without regard for the actual physical location. Using an automounter, the users and systems administrators can usually access files where they are "supposed to be" and find that they appear to be there.
Software
Tom Lyon developed the original automount software at Sun MicrosystemsSun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
: SunOS
SunOS
SunOS is a version of the Unix operating system developed by Sun Microsystems for their workstation and server computer systems. The SunOS name is usually only used to refer to versions 1.0 to 4.1.4 of SunOS...
4.0 made automounting available in 1988. Sun Microsystems eventually licensed this implementation to other commercial UNIX distributions. Solaris 2.0, first released in 1992, implemented its automounter with a pseudofilesystem called
autofs, which communicates with a user-mode daemon that performs mounts. Other Unix-likeUnix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems have adopted that implementation of the automounter - including AIX, HP-UX
HP-UX
HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...
, and Mac OS X 10.5 and later.
Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
has an independent implementation of an autofs-based automounter; version 5 of that automounter generally operates compatibly with the Solaris automounter.
In December 1989 Jan-Simon Pendry released amd, an automounter "based in spirit" on the SunOS automount program. amd has also become known as the Berkeley Automounter
Berkeley Automounter
The Berkeley Automounter first appeared in 4.4BSD, and is a computer automounter daemon. The original Berkeley automounter was created by Jan-Simon Pendry in 1989 and was donated to Berkeley...
.
Some operating systems also support automatic mounting of external drives (such as disk drives or flash drives that use FireWire or USB connections) and removable media (such as CDs and DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
s). This technology differs from the automounting described here; it involves mounting local media when the user attaches them to or inserts them into the system, rather than mounting directories from remote file servers when a reference is made to them. Linux currently (as of Linux 2.6) uses the user-space program udev
Udev
udev is the device manager for the Linux kernel. Primarily, it manages device nodes in /dev. It is the successor of devfs and hotplug, which means that it handles the /dev directory and all user space actions when adding/removing devices, including firmware load.-History:udev was new in Linux...
for this form of automounting. Some automounting functions have been implemented in the separate program HAL
HAL (software)
HAL was a software project providing a hardware abstraction layer for Unix-like computer systems.HAL is now deprecated on GNU/Linux systems, with functionality being merged into udev as of 2008–2010...
, but are being merged into udev. OpenBSD has hotplugd(8) which triggers special scripts on attach or detach of removable devices, so that user can easily add mounting of removable drives. In Mac OS X
diskarbitrationd carries out this form of automatic mounting.Disadvantages and caveats
While automounter utilities (and remote filesystems in general) can provide centrally managed, consistent and largely transparent access to an organization's storage services, they also can have their downsides:- Access to automounted directories can trigger delays while the automounter resolves the mapping and mounts the export into place.
- Timeouts can cause the unmounting of mounted directories (which situation can later result in mount delays upon the next attempted access).
- The mapping of mountpoint to export arguments is usually done via some directory service such as LDAPLightweight Directory Access ProtocolThe Lightweight Directory Access Protocol is an application protocol for accessing and maintaining distributed directory information services over an Internet Protocol network...
or NISNetwork Information ServiceThe Network Information Service, or NIS is a client–server directory service protocol for distributing system configuration data such as user and host names between computers on a computer network...
, which constitutes another dependency (potential point of failure). - When some systems require frequent access to some resources, while others only need occasional access, this can pose difficult or impossible problems in implementing a consistent, enterprise-wide mixture of locally "mirrored" (replicated) and automounted directories.
- When data is migrated from one file server (export) to another, there can be an indeterminate number of systems which, for various reasons, still have an active mount on the old location ("stale NFS mounts"); these can cause issues which may even necessitate the reboot of otherwise perfectly stable hosts.
- Organizations can find that they've created a "spaghetti" of mappings which can entail considerable management overhead and sometimes quite a bit of confusion among users and administrators.
- Users can become so accustomed to the transparency of automounted resources that they neglect to consider some of the differences in access semantics that may apply to networked filesystems, as compared to locally mounted devices. In particular, programmers may attempt to use "locking" techniques which are safe and provide the desired atomicity guarantees on local filesystems, but which are documented as inherently vulnerable to race conditionRace conditionA race condition or race hazard is a flaw in an electronic system or process whereby the output or result of the process is unexpectedly and critically dependent on the sequence or timing of other events...
s when used on NFS.
External links
- hotplug-diskmount using in conjunction with OpenBSD's hotplugd(8), automatically mounts removable disks to the filesystem