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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.
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.
Filesystem Size Used Avail Use% Mounted on tmpfs 256M 688K 256M 1% /tmp
On some Linux distributions (e.g. Debian, Ubuntu), /tmp is a normal directory, but /dev/shm uses tmpfs.
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
tmpfs is supported by the Linux kernel 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:
df -h mount -o remount,size=4G /tmp df -h
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.
DragonFly BSD has also ported NetBSD's implementation, where it is available in 2.5.1 and later versions.
OpenBSD ported NetBSD's tmpfs implementation as well, initially started by Pedro Martelletto and improved by many others. It has got linked to build on December, 17th 2013 (in time of post 5.4 release -current) and thus it will be shipped with OpenBSD 5.5.