Tmpfs¶
Tmpfs is a file system which keeps all of its files in virtual memory.
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, if swap was enabled for the tmpfs mount. tmpfs also supports THP.
tmpfs extends ramfs with a few userspace configurable options listed and explained further below, some of which can be reconfigured dynamically on the fly using a remount (‘mount -o remount ...’) of the filesystem. A tmpfs filesystem can be resized but it cannot be resized to a size below its current usage. tmpfs also supports POSIX ACLs, and extended attributes for the trusted.*, security.* and user.* namespaces. ramfs does not use swap and you cannot modify any parameter for a ramfs filesystem. The size limit of a ramfs filesystem is how much memory you have available, and so care must be taken if used so to not run out of memory.
An alternative to tmpfs and ramfs is to use brd to create RAM disks (/dev/ram*), which allows you to simulate a block device disk in physical RAM. To write data you would just then need to create an regular filesystem on top this ramdisk. As with ramfs, brd ramdisks cannot swap. brd ramdisks are also configured in size at initialization and you cannot dynamically resize them. Contrary to brd ramdisks, tmpfs has its own filesystem, it does not rely on the block layer at all.
Since tmpfs lives completely in the page cache and optionally 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:
size |
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. |
nr_blocks |
The same as size, but in blocks of PAGE_SIZE. |
nr_inodes |
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.
If nr_inodes is not 0, that limited space for inodes is also used up by extended attributes: “df -i“‘s IUsed and IUse% increase, IFree decreases.
tmpfs blocks may be swapped out, when there is a shortage of memory. tmpfs has a mount option to disable its use of swap:
noswap |
Disables swap. Remounts must respect the original settings. By default swap is enabled. |
tmpfs also supports Transparent Huge Pages which requires a kernel configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for your system (has_transparent_hugepage(), which is architecture specific). The mount options for this are:
huge=never |
Do not allocate huge pages. This is the default. |
huge=always |
Attempt to allocate huge page every time a new page is needed. |
huge=within_size |
Only allocate huge page if it will be fully within i_size. Also respect madvise(2) hints. |
huge=advise |
Only allocate huge page if requested with madvise(2). |
See also Transparent Hugepage Support, which describes the sysfs file /sys/kernel/mm/transparent_hugepage/shmem_enabled: which can be used to deny huge pages on all tmpfs mounts in an emergency, or to force huge pages on all tmpfs mounts for testing.
tmpfs also supports quota with the following mount options
quota |
User and group quota accounting and enforcement is enabled on the mount. Tmpfs is using hidden system quota files that are initialized on mount. |
usrquota |
User quota accounting and enforcement is enabled on the mount. |
grpquota |
Group quota accounting and enforcement is enabled on the mount. |
usrquota_block_hardlimit |
Set global user quota block hard limit. |
usrquota_inode_hardlimit |
Set global user quota inode hard limit. |
grpquota_block_hardlimit |
Set global group quota block hard limit. |
grpquota_inode_hardlimit |
Set global group quota inode hard limit. |
None of the quota related mount options can be set or changed on remount.
Quota limit parameters accept a suffix k, m or g for kilo, mega and giga and can’t be changed on remount. Default global quota limits are taking effect for any and all user/group/project except root the first time the quota entry for user/group/project id is being accessed - typically the first time an inode with a particular id ownership is being created after the mount. In other words, instead of the limits being initialized to zero, they are initialized with the particular value provided with these mount options. The limits can be changed for any user/group id at any time as they normally can be.
Note that tmpfs quotas do not support user namespaces so no uid/gid translation is done if quotas are enabled inside user namespaces.
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 ...’
mpol=default |
use the process allocation policy (see set_mempolicy(2)) |
mpol=prefer:Node |
prefers to allocate memory from the given Node |
mpol=bind:NodeList |
allocates memory only from nodes in NodeList |
mpol=interleave |
prefers to allocate from each node in turn |
mpol=interleave:NodeList |
allocates from each node of NodeList in turn |
mpol=local |
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:
mode |
The permissions as an octal number |
uid |
The user id |
gid |
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:
inode64 |
Use 64-bit inode numbers |
inode32 |
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.
tmpfs has the following mounting options for case-insensitive lookup support:
casefold |
Enable casefold support at this mount point using the given argument as the encoding standard. Currently only UTF-8 encodings are supported. If no argument is used, it will load the latest UTF-8 encoding available. |
strict_encoding |
Enable strict encoding at this mount point (disabled by default). In this mode, the filesystem refuses to create file and directory with names containing invalid UTF-8 characters. |
This option doesn’t render the entire filesystem case-insensitive. One needs to still set the casefold flag per directory, by flipping the +F attribute in an empty directory. Nevertheless, new directories will inherit the attribute. The mountpoint itself cannot be made case-insensitive.
Example:
$ mount -t tmpfs -o casefold=utf8-12.1.0,strict_encoding fs_name /mytmpfs
$ mount -t tmpfs -o casefold fs_name /mytmpfs
- Author:
Christoph Rohland <cr@sap.com>, 1.12.01
- Updated:
Hugh Dickins, 4 June 2007
- Updated:
KOSAKI Motohiro, 16 Mar 2010
- Updated:
Chris Down, 13 July 2020
- Updated:
André Almeida, 23 Aug 2024