tmpfs (short for Temporary File System) is a temporary file storage paradigm implemented in many Unix-like operating systems. It is intended to appear as a mounted file system, but data is 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 in tmpfs is temporary in the sense that no files will be created on your hard drive. If you unmount a tmpfs instance, everything stored therein is lost.
tmpfs puts everything into the kernel internal caches and grows and shrinks to accommodate the files it contains and is able to swap unneeded pages out to swap space. It has maximum size limits which can be adjusted on the fly via ‘mount -o remount …’
If you compare it to ramfs (which was the template to create tmpfs) you gain swapping and limit checking. Another similar thing is the RAM disk (/dev/ram*), which simulates a fixed size hard disk in physical RAM, where you have to create an ordinary filesystem on top. Ramdisks cannot swap and you do not have the possibility to resize them.
Since tmpfs lives completely in the page cache and on swap, all tmpfs pages will be shown as “Shmem” in /proc/meminfo and “Shared” in free(1). Notice that these counters also include shared memory (shmem, see ipcs(1)). The most reliable way to get the count is using df(1) and du(1).
tmpfs has the following uses:
- There is always a kernel internal mount which you will not see at all. This is used for shared anonymous mappings and SYSV shared memory.
This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not set, the user visible part of tmpfs is not built. But the internal mechanisms are always present.
- glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for POSIX shared memory (shm_open, shm_unlink). Adding the following line to /etc/fstab should take care of this:
tmpfs /dev/shm tmpfs defaults 0 0
Remember to create the directory that you intend to mount tmpfs on if necessary.
This mount is _not_ needed for SYSV shared memory. The internal mount is used for that. (In the 2.3 kernel versions it was necessary to mount the predecessor of tmpfs (shm fs) to use SYSV shared memory.)
- Some people (including me) find it very convenient to mount it e.g. on /tmp and /var/tmp and have a big swap partition. And now loop mounts of tmpfs files do work, so mkinitrd shipped by most distributions should succeed with a tmpfs /tmp.
- And probably a lot more I do not know about 🙂
tmpfs has three mount options for sizing:
||The limit of allocated bytes for this tmpfs instance. The default is half of your physical RAM without swap. If you oversize your tmpfs instances the machine will deadlock since the OOM handler will not be able to free that memory.
||The same as size, but in blocks of PAGE_SIZE.
||The maximum number of inodes for this instance. The default is half of the number of your physical RAM pages, or (on a machine with highmem) the number of lowmem RAM pages, whichever is the lower.
These parameters accept a suffix k, m or g for kilo, mega and giga and can be changed on remount. The size parameter also accepts a suffix % to limit this tmpfs instance to that percentage of your physical RAM: the default, when neither size nor nr_blocks is specified, is size=50%
If nr_blocks=0 (or size=0), blocks will not be limited in that instance; if nr_inodes=0, inodes will not be limited. It is generally unwise to mount with such options, since it allows any user with write access to use up all the memory on the machine; but enhances the scalability of that instance in a system with many CPUs making intensive use of it.
tmpfs has a mount option to set the NUMA memory allocation policy for all files in that instance (if CONFIG_NUMA is enabled) – which can be adjusted on the fly via ‘mount -o remount …’
||use the process allocation policy (see set_mempolicy(2))
||prefers to allocate memory from the given Node
||allocates memory only from nodes in NodeList
||prefers to allocate from each node in turn
||allocates from each node of NodeList in turn
||prefers to allocate memory from the local node
NodeList format is a comma-separated list of decimal numbers and ranges, a range being two hyphen-separated decimal numbers, the smallest and largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15
A memory policy with a valid NodeList will be saved, as specified, for use at file creation time. When a task allocates a file in the file system, the mount option memory policy will be applied with a NodeList, if any, modified by the calling task’s cpuset constraints [See CPUSETS] and any optional flags, listed below. If the resulting NodeLists is the empty set, the effective memory policy for the file will revert to “default” policy.
NUMA memory allocation policies have optional flags that can be used in conjunction with their modes. These optional flags can be specified when tmpfs is mounted by appending them to the mode before the NodeList. See NUMA Memory Policy for a list of all available memory allocation policy mode flags and their effect on memory policy.
=static is equivalent to MPOL_F_STATIC_NODES
=relative is equivalent to MPOL_F_RELATIVE_NODES
For example, mpol=bind=static:NodeList, is the equivalent of an allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
Note that trying to mount a tmpfs with an mpol option will fail if the running kernel does not support NUMA; and will fail if its nodelist specifies a node which is not online. If your system relies on that tmpfs being mounted, but from time to time runs a kernel built without NUMA capability (perhaps a safe recovery kernel), or with fewer nodes online, then it is advisable to omit the mpol option from automatic mount options. It can be added later, when the tmpfs is already mounted on MountPoint, by ‘mount -o remount,mpol=Policy:NodeList MountPoint’.
To specify the initial root directory you can use the following mount options:
||The permissions as an octal number
||The user id
||The group id
These options do not have any effect on remount. You can change these parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode numbers:
||Use 64-bit inode numbers
||Use 32-bit inode numbers
On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time. On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default. inode64 avoids the possibility of multiple files with the same inode number on a single device; but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached – if a long-lived tmpfs is accessed by 32-bit applications so ancient that opening a file larger than 2GiB fails with EINVAL.
So ‘mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs’ will give you tmpfs instance on /mytmpfs which can allocate 10GB RAM/SWAP in 10240 inodes and it is only accessible by root.
Everything stored in tmpfs is temporary in the sense that no files will be directly created on non-volatile storage such as a hard drive (although 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.
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
Some Linux distributions (e.g. Debian) do not have a tmpfs mounted on /tmp by default; in this case, files under /tmp will be stored in the same file system as /.
And on almost all Linux distributions, a tmpfs is mounted on /run/ or /var/run/ to store temporary run-time files such as PID files and Unix domain sockets. Temporary system files such as firmware variables are stored in
There are several independent variants of the tmpfs concept. One of the earliest was developed by Sun Microsystems for SunOS, and other operating systems like the BSDs and Linux provided their own.
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 December 1992. Output for the Solaris
df command will show swap as the background storage 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 beginning in version 2.4. Linux 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).
4.2BSD introduced MFS, a memory-based file system implemented by applying the existing 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 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.
OpenBSD ported NetBSD’s tmpfs implementation as well, initially started by Pedro Martelletto and improved by many others. It was enabled in builds from December 17, 2013. The first release of OpenBSD with tmpfs included was 5.5. OpenBSD 6.0 disabled tmpfs due to lack of maintenance.
Due to the higher speeds of RAM compared to disk storage, tmpfs allows cache to be much faster when stored in one, leading to a more efficient overall system. Since RAM is cleared upon reboot, tmpfs prevents systems from becoming too cluttered without requiring that the user manually deletes temporary files. In addition, storing files in RAM prevents disks from filling up too quickly and extends the life of solid-state drives by reducing the amount of writes.
On systems with less RAM, a tmpfs will fill up the memory quickly.
If cache files are stored in tmpfs, programs will lose their cached data across reboots.
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