man > tmpfs(5)

tmpfs(5)                                 File Formats Manual                                tmpfs(5)

NAME
       tmpfs - a virtual memory filesystem

DESCRIPTION
       The  tmpfs  facility allows the creation of filesystems whose contents reside in virtual mem‐
       ory.  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 limit is removed if the size is 0.

              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.  The limit is removed if the number is 0.

              Inodes may be specified with k, m, or g suffixes like size, but not a % suffix.

       noswap(since Linux 6.4)
              Disables  swap.   Remounts must respect the original settings.  By default swap is en‐
              abled.

       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  CON‐
              FIG_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 fad‐
                     vise(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 small‐
              est 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  attrib‐
       utes 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/ad‐
       min-guide/mm/transhuge.rst.

Linux man-pages 6.7                          2023-10-31                                     tmpfs(5)