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 (/dev/shm is automagically created if you use devfs).
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 a 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
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