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tmpfs(5)
TMPFS(5) Linux Programmer's Manual TMPFS(5)
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 typi‐
cally 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 filesys‐
tem).
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, 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 fad‐
vise(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 test‐
ing.
mpol=mpol_option (since Linux 2.6.15)
Set the NUMA memory allocation policy for all files in this in‐
stance (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 num‐
bers 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 filesys‐
tems, 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 Docu‐
mentation/admin-guide/mm/transhuge.rst.
COLOPHON
This page is part of release 5.02 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/.
Linux 2019-03-06 TMPFS(5)