Section (5) tmpfs
Name
tmpfs — a virtual memory filesystem
DESCRIPTION
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).
Mount options
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
, org
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
nornr_blocks
is specified, issize=50%
. nr_blocks
=blocks
-
The same as
size
, but in blocks ofPAGE_CACHE_SIZE
.Blocks may be specified with
k
,m
, org
suffixes likesize
, 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
, org
suffixes likesize
, 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
.
VERSIONS
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.
NOTES
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
.
SEE ALSO
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
.
COLOPHON
This page is part of release 5.04 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/.
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