tmpfs — a virtual memory filesystem
The tmpfs
facility allows the creation of filesystems whose contents
reside in virtual memory. Since the files on such filesystems
typically reside in RAM, file access is extremely fast.
The filesystem is automatically created when mounting a
filesystem with the type tmpfs
via a command such as
the following:
$ sudo mount −t tmpfs −o size=10M tmpfs /mnt/mytmpfs
A tmpfs
filesystem has the following properties:
The filesystem can employ swap space when physical memory pressure demands it.
The filesystem consumes only as much physical memory and swap space as is required to store the current contents of the filesystem.
During a remount operation (mount −o remount), the filesystem size can be changed (without losing the existing contents of the filesystem).
If a tmpfs
filesystem is unmounted, its contents are discarded
(lost).
The tmpfs
filesystem supports the following mount options:
size
=bytes
Specify an upper limit on the size of the filesystem. The size is given in bytes, and rounded up to entire pages.
The size may have a k
, m
,
or g
suffix for Ki, Mi,
Gi (binary kilo (kibi), binary mega (mebi), and
binary giga (gibi)).
The size may also have a % suffix to limit this instance to a percentage of physical RAM.
The default, when neither size
nor nr_blocks
is
specified, is size=50%
.
nr_blocks
=blocks
The same as size
, but in blocks
of PAGE_CACHE_SIZE
.
Blocks may be specified with k
, m
,
or g
suffixes like
size
, but
not a % suffix.
nr_inodes
=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 smaller.
Inodes may be specified with k
, m
,
or g
suffixes like
size
, but
not a % suffix.
mode
=mode
Set initial permissions of the root directory.
gid
=gid
(since Linux
2.5.7)Set the initial group ID of the root directory.
uid
=uid
(since Linux
2.5.7)Set the initial user ID of the root directory.
huge
=huge_option
(since
Linux 4.7.0)Set the huge table memory allocation policy for
all files in this instance (if CONFIG_TRANSPARENT_HUGE_PAGECACHE
is enabled).
The huge_option
value is
one of the following:
- never
Do not allocate huge pages. This is the default.
- always
Attempt to allocate huge pages every time a new page is needed.
- within_size
Only allocate huge page if it will be fully within
i_size
. Also respect fadvise(2)/madvise(2) hints- advise
Only allocate huge pages if requested with fadvise(2)/madvise(2).
- deny
For use in emergencies, to force the huge option off from all mounts.
- force
Force the huge option on for all mounts; useful for testing.
mpol
=mpol_option
(since
Linux 2.6.15)Set the NUMA memory allocation policy for all
files in this instance (if CONFIG_NUMA
is enabled).
The mpol_option
value is
one of the following:
- default
Use the process allocation policy (see set_mempolicy(2)).
prefer
:node
Preferably allocate memory from the given
node
.bind
:nodelist
Allocate memory only from nodes in
nodelist
.- interleave
Allocate from each node in turn.
interleave
:nodelist
Allocate from each node of
in
turn.- local
Preferably allocate memory from the local node.
In the above, nodelist
is a
comma-separated list of decimal numbers and ranges
that specify NUMA nodes. A range is a pair of
hyphen-separated decimal numbers, the smallest and
largest node numbers in the range. For example,
mpol=bind:0−3,5,7,9−15
.
The tmpfs
facility was added in Linux 2.4, as a successor to the older
ramfs
facility,
which did not provide limit checking or allow for the use of
swap space.
In order for user-space tools and applications to create
tmpfs
filesystems,
the kernel must be configured with the CONFIG_TMPFS
option.
The tmpfs
filesystem supports extended attributes (see xattr(7)), but user
extended attributes are
not permitted.
An internal shared memory filesystem is used for System V
shared memory (shmget(2)) and shared
anonymous mappings (mmap(2) with the
MAP_SHARED
and MAP_ANONYMOUS
flags). This filesystem is
available regardless of whether the kernel was configured
with the CONFIG_TMPFS
option.
A tmpfs
filesystem mounted at /dev/shm
is used for the implementation of POSIX shared memory
(shm_overview(7)) and POSIX
semaphores (sem_overview(7)).
The amount of memory consumed by all tmpfs
filesystems is shown in
the Shmem
field of
/proc/meminfo
and in the
shared
field
displayed by free(1).
The tmpfs
facility was formerly called shmfs
.
df(1), du(1), memfd_create(2), mmap(2), set_mempolicy(2), shm_open(3), mount(8)
The kernel source files Documentation/filesystems/tmpfs.txt
and
Documentation/admin−guide/mm/transhuge.rst
.
This page is part of release 5.11 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 2016 by Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |