Documentation/filesystems/tmpfs.txt

   1 Tmpfs is a file system which keeps all files in virtual memory.
   2
   3
   4 Everything in tmpfs is temporary in the sense that no files will be
   5 created on your hard drive. If you unmount a tmpfs instance,
   6 everything stored therein is lost.
   7
   8 tmpfs puts everything into the kernel internal caches and grows and
   9 shrinks to accommodate the files it contains and is able to swap
  10 unneeded pages out to swap space. It has maximum size limits which can
  11 be adjusted on the fly via 'mount -o remount ...'
  12
  13 If you compare it to ramfs (which was the template to create tmpfs)
  14 you gain swapping and limit checking. Another similar thing is the RAM
  15 disk (/dev/ram*), which simulates a fixed size hard disk in physical
  16 RAM, where you have to create an ordinary filesystem on top. Ramdisks
  17 cannot swap and you do not have the possibility to resize them.
  18
  19 Since tmpfs lives completely in the page cache and on swap, all tmpfs
  20 pages currently in memory will show up as cached. It will not show up
  21 as shared or something like that. Further on you can check the actual
  22 RAM+swap use of a tmpfs instance with df(1) and du(1).
  23
  24
  25 tmpfs has the following uses:
  26
  27 1) There is always a kernel internal mount which you will not see at
  28    all. This is used for shared anonymous mappings and SYSV shared
  29    memory.
  30
  31    This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
  32    set, the user visible part of tmpfs is not build. But the internal
  33    mechanisms are always present.
  34
  35 2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
  36    POSIX shared memory (shm_open, shm_unlink). Adding the following
  37    line to /etc/fstab should take care of this:
  38
  39         tmpfs   /dev/shm        tmpfs   defaults        0 0
  40
  41    Remember to create the directory that you intend to mount tmpfs on
  42    if necessary.
  43
  44    This mount is _not_ needed for SYSV shared memory. The internal
  45    mount is used for that. (In the 2.3 kernel versions it was
  46    necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
  47    shared memory)
  48
  49 3) Some people (including me) find it very convenient to mount it
  50    e.g. on /tmp and /var/tmp and have a big swap partition. And now
  51    loop mounts of tmpfs files do work, so mkinitrd shipped by most
  52    distributions should succeed with a tmpfs /tmp.
  53
  54 4) And probably a lot more I do not know about :-)
  55
  56
  57 tmpfs has three mount options for sizing:
  58
  59 size:      The limit of allocated bytes for this tmpfs instance. The
  60            default is half of your physical RAM without swap. If you
  61            oversize your tmpfs instances the machine will deadlock
  62            since the OOM handler will not be able to free that memory.
  63 nr_blocks: The same as size, but in blocks of PAGE_CACHE_SIZE.
  64 nr_inodes: The maximum number of inodes for this instance. The default
  65            is half of the number of your physical RAM pages, or (on a
  66            machine with highmem) the number of lowmem RAM pages,
  67            whichever is the lower.
  68
  69 These parameters accept a suffix k, m or g for kilo, mega and giga and
  70 can be changed on remount.  The size parameter also accepts a suffix %
  71 to limit this tmpfs instance to that percentage of your physical RAM:
  72 the default, when neither size nor nr_blocks is specified, is size=50%
  73
  74 If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
  75 if nr_inodes=0, inodes will not be limited.  It is generally unwise to
  76 mount with such options, since it allows any user with write access to
  77 use up all the memory on the machine; but enhances the scalability of
  78 that instance in a system with many cpus making intensive use of it.
  79
  80
  81 tmpfs has a mount option to set the NUMA memory allocation policy for
  82 all files in that instance (if CONFIG_NUMA is enabled) - which can be
  83 adjusted on the fly via 'mount -o remount ...'
  84
  85 mpol=default             prefers to allocate memory from the local node
  86 mpol=prefer:Node         prefers to allocate memory from the given Node
  87 mpol=bind:NodeList       allocates memory only from nodes in NodeList
  88 mpol=interleave          prefers to allocate from each node in turn
  89 mpol=interleave:NodeList allocates from each node of NodeList in turn
  90
  91 NodeList format is a comma-separated list of decimal numbers and ranges,
  92 a range being two hyphen-separated decimal numbers, the smallest and
  93 largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15
  94
  95 Note that trying to mount a tmpfs with an mpol option will fail if the
  96 running kernel does not support NUMA; and will fail if its nodelist
  97 specifies a node >= MAX_NUMNODES.  If your system relies on that tmpfs
  98 being mounted, but from time to time runs a kernel built without NUMA
  99 capability (perhaps a safe recovery kernel), or configured to support
 100 fewer nodes, then it is advisable to omit the mpol option from automatic
 101 mount options.  It can be added later, when the tmpfs is already mounted
 102 on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
 103
 104
 105 To specify the initial root directory you can use the following mount
 106 options:
 107
 108 mode:   The permissions as an octal number
 109 uid:    The user id
 110 gid:    The group id
 111
 112 These options do not have any effect on remount. You can change these
 113 parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
 114
 115
 116 So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
 117 will give you tmpfs instance on /mytmpfs which can allocate 10GB
 118 RAM/SWAP in 10240 inodes and it is only accessible by root.
 119
 120
 121 Author:
 122    Christoph Rohland <cr@sap.com>, 1.12.01
 123 Updated:
 124    Hugh Dickins <hugh@veritas.com>, 19 February 2006