man5/tmpfs.5

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  25 .TH TMPFS 5 2016-11-07 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 tmpfs \- a virtual memory filesystem
  28 .SH DESCRIPTION
  29 The
  30 .B tmpfs
  31 facility allows the creation of filesystems whose contents reside
  32 in virtual memory.
  33 Since the files on such filesystems typically reside in RAM,
  34 file access is extremely fast.
  35
  36 The filesystem is automatically created when mounting
  37 a filesystem with the type
  38 .BR tmpfs
  39 via a command such as the following:
  40
  41     $ sudo mount \-t tmpfs -o size=10M tmpfs /mnt/mytmpfs
  42
  43 A
  44 .B tmpfs
  45 filesystem has the following properties:
  46 .IP * 3
  47 The filesystem can employ swap space when physical memory pressure
  48 demands it.
  49 .IP *
  50 The
  51 .I size
  52 option can be used to specify an upper limit on the size of the filesystem.
  53 (The default size is half of the available RAM size.)
  54 The filesystem consumes only as much physical memory and swap space
  55 as is required to store the current contents of the filesystem.
  56 .IP *
  57 During a remount operation
  58 .RI ( "mount\ \-o\ remount" ),
  59 the filesystem size can be changed
  60 (without losing the existing contents of the filesystem).
  61 .PP
  62 If a
  63 .B tmpfs
  64 filesystem is unmounted, its contents are discarded (lost).
  65 .SH VERSIONS
  66 The
  67 .B tmpfs
  68 facility was added in Linux 2.4, as a successor to the older
  69 .B ramfs
  70 facility, which did not provide limit checking or
  71 allow for the use of swap space.
  72 .SH NOTES
  73 For a description of the mount options that may be employed when mounting a
  74 .B tmpfs
  75 filesystem, see
  76 .BR mount (8).
  77
  78 In order to for user-space tools and applications to create
  79 .B tmpfs
  80 filesystems, the kernel must be configured with the
  81 .B CONFIG_TMPFS
  82 option.
  83
  84 An internal shared memory filesystem is used for
  85 System V shared memory
  86 .BR (shmget (2))
  87 and shared anonymous mappings
  88 .RB ( mmap (2)
  89 with the
  90 .B MAP_SHARED
  91 and
  92 .BR MAP_ANONYMOUS
  93 flags).
  94 This filesystem is available regardless of whether
  95 the kernel was configured with the
  96 .B CONFIG_TMPFS
  97 option.
  98
  99 A
 100 .B tmpfs
 101 filesystem mounted at
 102 .IR /dev/shm
 103 as used for the implementation of POSIX shared memory
 104 .RB ( shm_overview (7))
 105 and POSIX semaphores
 106 .RB ( sem_overview (7)).
 107
 108 The amount of memory consumed by all
 109 .B tmpfs
 110 filesystems is shown in the
 111 .I Shmem
 112 field of
 113 .IR /proc/meminfo
 114 and in the
 115 .I shared
 116 field displayed by
 117 .BR free (1).
 118
 119 The
 120 .B tmpfs
 121 facility was formerly called
 122 .BR shmfs .
 123 .SH SEE ALSO
 124 .BR df (1),
 125 .BR du (1),
 126 .BR mount (8)
 127 .PP
 128 The kernel source file
 129 .IR Documentation/filesystems/tmpfs.txt .