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 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.

Linux								    2019-03-06								  TMPFS(5)