TMPFS
Encyclopedia
tmpfs is a common name for a temporary file storage facility on many Unix-like
operating systems. It is intended to appear as a mounted file system
, but stored in volatile memory
instead of a persistent storage device. A similar construction is a RAM disk
, which appears as a virtual disk drive and hosts a disk file system.
The memory used by tmpfs grows and shrinks to accommodate the files it contains and can be swapped out to swap space.
Many Unix distributions enable and use tmpfs by default for the /tmp branch of the file system or for shared memory
. This can be observed with df
as in this example:
Filesystem Size Used Avail Use% Mounted on
tmpfs 256M 688K 256M 1% /tmp
4 includes what is most likely the earliest implementation of tmpfs; it first appeared in SunOS 4.0 in late 1987, together with new orthogonal address space management that allowed any object to be memory mapped.
The Solaris /tmp directory was made a tmpfs file system by default starting with Solaris 2.1, released in November 1994. Output for the Solaris
# df -k
Filesystem kbytes used avail capacity Mounted on
swap 601592 0 601592 0% /tmp/test
from version 2.4 and up. tmpfs (previously known as shmfs) is based on the ramfs code used during bootup and also uses the page cache, but unlike ramfs it supports swapping out less-used pages to swap space as well as filesystem size and inode limits to prevent out of memory situations (defaulting to half of physical RAM and half the number of RAM pages, respectively). These options are set at mount time and may be modified by remounting the filesystem.
disk filesystem to a virtual memory region.
tmpfs, a memory filesystem implemented using conventional in-memory data structures in order to improve on the performance of MFS, was merged into the official NetBSD
source tree on September 10, 2005; it is available in 4.0 and later versions.
FreeBSD
has ported NetBSD's implementation, where it is available in 7.0 and later versions. DragonFly BSD
has also ported NetBSD's implementation, where it is available in 2.5.1 and later versions.
Unix-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....
operating systems. It is intended to appear as a mounted file system
File system
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 which contain it. A file system organizes data in an efficient manner and is tuned to the...
, but stored in volatile memory
Volatile memory
Volatile memory, also known as volatile storage, is computer memory that requires power to maintain the stored information, unlike non-volatile memory which does not require a maintained power supply...
instead of a persistent storage device. A similar construction is a RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
, which appears as a virtual disk drive and hosts a disk file system.
Semantics
Everything stored in tmpfs is temporary in the sense that no files will be created on the hard drive; however, swap space is used as backing store in case of low memory situations. On reboot, everything in tmpfs will be lost.The memory used by tmpfs grows and shrinks to accommodate the files it contains and can be swapped out to swap space.
Many Unix distributions enable and use tmpfs by default for the /tmp branch of the file system or for shared memory
Shared memory
In computing, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors...
. This can be observed with df
Df (Unix)
df is a standard Unix computer program used to display the amount of available disk space for filesystems on which the invoking user has appropriate read access...
as in this example:
Filesystem Size Used Avail Use% Mounted on
tmpfs 256M 688K 256M 1% /tmp
SunOS/Solaris
SunOSSunOS
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 includes what is most likely the earliest implementation of tmpfs; it first appeared in SunOS 4.0 in late 1987, together with new orthogonal address space management that allowed any object to be memory mapped.
The Solaris /tmp directory was made a tmpfs file system by default starting with Solaris 2.1, released in November 1994. Output for the Solaris
df command will show swap as the filesystem type for any tmpfs volume:# df -k
Filesystem kbytes used avail capacity Mounted on
swap 601592 0 601592 0% /tmp/test
Linux
tmpfs is supported by the Linux kernelLinux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
from version 2.4 and up. tmpfs (previously known as shmfs) is based on the ramfs code used during bootup and also uses the page cache, but unlike ramfs it supports swapping out less-used pages to swap space as well as filesystem size and inode limits to prevent out of memory situations (defaulting to half of physical RAM and half the number of RAM pages, respectively). These options are set at mount time and may be modified by remounting the filesystem.
BSD
4.2BSD introduced MFS, a memory-based file system implemented by applying the existing FFSUnix File System
The Unix file system is a file system used by many Unix and Unix-like operating systems. It is also called the Berkeley Fast File System, the BSD Fast File System or FFS...
disk filesystem to a virtual memory region.
tmpfs, a memory filesystem implemented using conventional in-memory data structures in order to improve on the performance of MFS, was merged into the official NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
source tree on September 10, 2005; it is available in 4.0 and later versions.
FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
has ported NetBSD's implementation, where it is available in 7.0 and later versions. DragonFly BSD
DragonFly BSD
DragonFly BSD is a free Unix-like operating system created as a fork of FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and a FreeBSD developer between 1994 and 2003, began work on DragonFly BSD in June 2003 and announced it on the FreeBSD mailing lists on July...
has also ported NetBSD's implementation, where it is available in 2.5.1 and later versions.
External links
- Documentation/filesystems/tmpfs.txt documentation in Linux source tree
- tmpfs(7FS) Solaris 10 man page
- mount_tmpfs(8) NetBSD man page