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Linux/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             use the process allocation policy
 86                          (see set_mempolicy(2))
 87 mpol=prefer:Node         prefers to allocate memory from the given Node
 88 mpol=bind:NodeList       allocates memory only from nodes in NodeList
 89 mpol=interleave          prefers to allocate from each node in turn
 90 mpol=interleave:NodeList allocates from each node of NodeList in turn
 91 mpol=local               prefers to allocate memory from the local node
 92 
 93 NodeList format is a comma-separated list of decimal numbers and ranges,
 94 a range being two hyphen-separated decimal numbers, the smallest and
 95 largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15
 96 
 97 A memory policy with a valid NodeList will be saved, as specified, for
 98 use at file creation time.  When a task allocates a file in the file
 99 system, the mount option memory policy will be applied with a NodeList,
100 if any, modified by the calling task's cpuset constraints
101 [See Documentation/cgroups/cpusets.txt] and any optional flags, listed
102 below.  If the resulting NodeLists is the empty set, the effective memory
103 policy for the file will revert to "default" policy.
104 
105 NUMA memory allocation policies have optional flags that can be used in
106 conjunction with their modes.  These optional flags can be specified
107 when tmpfs is mounted by appending them to the mode before the NodeList.
108 See Documentation/vm/numa_memory_policy.txt for a list of all available
109 memory allocation policy mode flags and their effect on memory policy.
110 
111         =static         is equivalent to        MPOL_F_STATIC_NODES
112         =relative       is equivalent to        MPOL_F_RELATIVE_NODES
113 
114 For example, mpol=bind=static:NodeList, is the equivalent of an
115 allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
116 
117 Note that trying to mount a tmpfs with an mpol option will fail if the
118 running kernel does not support NUMA; and will fail if its nodelist
119 specifies a node which is not online.  If your system relies on that
120 tmpfs being mounted, but from time to time runs a kernel built without
121 NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
122 online, then it is advisable to omit the mpol option from automatic
123 mount options.  It can be added later, when the tmpfs is already mounted
124 on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
125 
126 
127 To specify the initial root directory you can use the following mount
128 options:
129 
130 mode:   The permissions as an octal number
131 uid:    The user id 
132 gid:    The group id
133 
134 These options do not have any effect on remount. You can change these
135 parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
136 
137 
138 So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
139 will give you tmpfs instance on /mytmpfs which can allocate 10GB
140 RAM/SWAP in 10240 inodes and it is only accessible by root.
141 
142 
143 Author:
144    Christoph Rohland <cr@sap.com>, 1.12.01
145 Updated:
146    Hugh Dickins, 4 June 2007
147 Updated:
148    KOSAKI Motohiro, 16 Mar 2010

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