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NAME | DESCRIPTION | VERSIONS | NOTES | SEE ALSO |
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tmpfs(5) File Formats Manual tmpfs(5)
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).
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_HUGEPAGE 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) and madvise(2)
hints
advise Only allocate huge pages if requested with
fadvise(2) or 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.
Linux man-pages (unreleased) (date) tmpfs(5)
Pages that refer to this page: fallocate(2), fanotify_mark(2), ioctl_userfaultfd(2), lseek(2), madvise(2), memfd_create(2), mmap(2), remap_file_pages(2), seccomp_unotify(2), swapon(2), shm_open(3), filesystems(5), namespace.conf(5), proc(5), sysfs(5), cgroups(7), keyrings(7), shm_overview(7), user_namespaces(7), mount(8), systemd-fstab-generator(8)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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