man5/tmpfs.5

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  25 .TH TMPFS 5 2017-05-03 "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 .PP
  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 .PP
  41 .in +4n
  42 .EX
  43 $ sudo mount \-t tmpfs -o size=10M tmpfs /mnt/mytmpfs
  44 .EE
  45 .in
  46 .PP
  47 A
  48 .B tmpfs
  49 filesystem has the following properties:
  50 .IP * 3
  51 The filesystem can employ swap space when physical memory pressure
  52 demands it.
  53 .IP *
  54 The
  55 .I size
  56 option can be used to specify an upper limit on the size of the filesystem.
  57 (The default size is half of the available RAM size.)
  58 The filesystem consumes only as much physical memory and swap space
  59 as is required to store the current contents of the filesystem.
  60 .IP *
  61 During a remount operation
  62 .RI ( "mount\ \-o\ remount" ),
  63 the filesystem size can be changed
  64 (without losing the existing contents of the filesystem).
  65 .PP
  66 If a
  67 .B tmpfs
  68 filesystem is unmounted, its contents are discarded (lost).
  69 .SH VERSIONS
  70 The
  71 .B tmpfs
  72 facility was added in Linux 2.4, as a successor to the older
  73 .B ramfs
  74 facility, which did not provide limit checking or
  75 allow for the use of swap space.
  76 .SH NOTES
  77 For a description of the mount options that may be employed when mounting a
  78 .B tmpfs
  79 filesystem, see
  80 .BR mount (8).
  81 .PP
  82 In order for user-space tools and applications to create
  83 .B tmpfs
  84 filesystems, the kernel must be configured with the
  85 .B CONFIG_TMPFS
  86 option.
  87 .PP
  88 The
  89 .BR tmpfs
  90 filesystem supports extended attributes (see
  91 .BR xattr (7)),
  92 but
  93 .I user
  94 extended attributes are not permitted.
  95 .PP
  96 An internal shared memory filesystem is used for
  97 System V shared memory
  98 .RB ( shmget (2))
  99 and shared anonymous mappings
 100 .RB ( mmap (2)
 101 with the
 102 .B MAP_SHARED
 103 and
 104 .BR MAP_ANONYMOUS
 105 flags).
 106 This filesystem is available regardless of whether
 107 the kernel was configured with the
 108 .B CONFIG_TMPFS
 109 option.
 110 .PP
 111 A
 112 .B tmpfs
 113 filesystem mounted at
 114 .IR /dev/shm
 115 as used for the implementation of POSIX shared memory
 116 .RB ( shm_overview (7))
 117 and POSIX semaphores
 118 .RB ( sem_overview (7)).
 119 .PP
 120 The amount of memory consumed by all
 121 .B tmpfs
 122 filesystems is shown in the
 123 .I Shmem
 124 field of
 125 .IR /proc/meminfo
 126 and in the
 127 .I shared
 128 field displayed by
 129 .BR free (1).
 130 .PP
 131 The
 132 .B tmpfs
 133 facility was formerly called
 134 .BR shmfs .
 135 .SH SEE ALSO
 136 .BR df (1),
 137 .BR du (1),
 138 .BR memfd_create (2),
 139 .BR mmap (2),
 140 .BR shm_open (3),
 141 .BR mount (8)
 142 .PP
 143 The kernel source file
 144 .IR Documentation/filesystems/tmpfs.txt .