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 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 .\" See mm/shmem.c:shmem_parse_options for options it supports.
  66 .SS Mount options
  67 The
  68 .B tmpfs
  69 filesystem supports the following mount options:
  70 .TP
  71 .BR size "=\fIbytes\fP"
  72 Specify an upper limit on the size of the filesystem.
  73 The size is given in bytes, and rounded up to entire pages.
  74 .IP
  75 The size may have a
  76 .BR k ,
  77 .BR m ,
  78 or
  79 .B g
  80 suffix for Ki, Mi, Gi (binary kilo (kibi), binary mega (mebi) and binary giga
  81 (gibi)).
  82 .IP
  83 The size may also have a % suffix to limit this instance to a percentage of
  84 physical RAM.
  85 .IP
  86 The default, when neither
  87 .B size
  88 nor
  89 .B nr_blocks
  90 is specified, is
  91 .IR size=50% .
  92 .TP
  93 .BR nr_blocks "=\fIblocks\fP"
  94 The same as
  95 .BR size ,
  96 but in blocks of
  97 .BR PAGE_CACHE_SIZE .
  98 .IP
  99 Blocks may be specified with
 100 .BR k ,
 101 .BR m ,
 102 or
 103 .B g
 104 suffixes like
 105 .BR size ,
 106 but not a % suffix.
 107 .TP
 108 .BR nr_inodes "=\fIinodes\fP"
 109 The maximum number of inodes for this instance.
 110 The default is half of the number of your physical RAM pages, or (on a
 111 machine with highmem) the number of lowmem RAM pages, whichever is smaller.
 112 .IP
 113 Inodes may be specified with
 114 .BR k ,
 115 .BR m ,
 116 or
 117 .B g
 118 suffixes like
 119 .BR size ,
 120 but not a % suffix.
 121 .TP
 122 .BR mode "=\fImode\fP"
 123 Set initial permissions of the root directory.
 124 .TP
 125 .BR gid "=\fIgid\fP (since Linux 2.5.7)"
 126 .\" Technically this is also in some version of Linux 2.4.
 127 .\" commit 099445b489625b80b1d6687c9b6072dbeaca4096
 128 Set the initial group ID of the root directory.
 129 .TP
 130 .BR uid "=\fIuid\fP (since Linux 2.5.7)"
 131 .\" Technically this is also in some version of Linux 2.4.
 132 .\" commit 099445b489625b80b1d6687c9b6072dbeaca4096
 133 Set the initial user ID of the root directory.
 134 .TP
 135 .BR huge "=\fIhuge_option\fR (since Linux 4.7.0)"
 136 .\" commit 5a6e75f8110c97e2a5488894d4e922187e6cb343
 137 Set the huge table memory allocation policy for all files in this instance (if
 138 .B CONFIG_TRANSPARENT_HUGE_PAGECACHE
 139 is enabled).
 140 .IP
 141 The
 142 .I huge_option
 143 value is one of the following:
 144 .RS
 145 .TP
 146 .B never
 147
 148 .TP
 149 .B always
 150
 151 .TP
 152 .B within_size
 153
 154 .TP
 155 .B advise
 156
 157 .TP
 158 .B deny
 159
 160 .TP
 161 .B force
 162
 163 .RE
 164 .TP
 165 .BR mpol "=\fImpol_option\fR (since Linux 2.6.15)"
 166 .\" commit 7339ff8302fd70aabf5f1ae26e0c4905fa74a495
 167 Set the NUMA memory allocation policy for all files in this instance (if
 168 .B CONFIG_NUMA
 169 is enabled).
 170 .IP
 171 The
 172 .I mpol_option
 173 value is one of the following:
 174 .RS
 175 .TP
 176 .B default
 177
 178 .TP
 179 .BR prefer ":\fINode\fP"
 180
 181 .TP
 182 .BR bind ":\fINodeList\fP"
 183
 184 .TP
 185 .B interleave
 186
 187 .TP
 188 .BR interleave ":\fINodeList\fP"
 189
 190 .RE
 191 .SH VERSIONS
 192 The
 193 .B tmpfs
 194 facility was added in Linux 2.4, as a successor to the older
 195 .B ramfs
 196 facility, which did not provide limit checking or
 197 allow for the use of swap space.
 198 .SH NOTES
 199 In order for user-space tools and applications to create
 200 .B tmpfs
 201 filesystems, the kernel must be configured with the
 202 .B CONFIG_TMPFS
 203 option.
 204 .PP
 205 The
 206 .BR tmpfs
 207 filesystem supports extended attributes (see
 208 .BR xattr (7)),
 209 but
 210 .I user
 211 extended attributes are not permitted.
 212 .PP
 213 An internal shared memory filesystem is used for
 214 System V shared memory
 215 .RB ( shmget (2))
 216 and shared anonymous mappings
 217 .RB ( mmap (2)
 218 with the
 219 .B MAP_SHARED
 220 and
 221 .BR MAP_ANONYMOUS
 222 flags).
 223 This filesystem is available regardless of whether
 224 the kernel was configured with the
 225 .B CONFIG_TMPFS
 226 option.
 227 .PP
 228 A
 229 .B tmpfs
 230 filesystem mounted at
 231 .IR /dev/shm
 232 as used for the implementation of POSIX shared memory
 233 .RB ( shm_overview (7))
 234 and POSIX semaphores
 235 .RB ( sem_overview (7)).
 236 .PP
 237 The amount of memory consumed by all
 238 .B tmpfs
 239 filesystems is shown in the
 240 .I Shmem
 241 field of
 242 .IR /proc/meminfo
 243 and in the
 244 .I shared
 245 field displayed by
 246 .BR free (1).
 247 .PP
 248 The
 249 .B tmpfs
 250 facility was formerly called
 251 .BR shmfs .
 252 .SH SEE ALSO
 253 .BR df (1),
 254 .BR du (1),
 255 .BR memfd_create (2),
 256 .BR mmap (2),
 257 .BR shm_open (3),
 258 .BR mount (8)
 259 .PP
 260 The kernel source file
 261 .IR Documentation/filesystems/tmpfs.txt .