noble (5) tmpfs.5.gz

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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 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 enabled.

       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.

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.