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1Tmpfs is a file system which keeps all files in virtual memory.
2
3
4Everything in tmpfs is temporary in the sense that no files will be
5created on your hard drive. If you unmount a tmpfs instance,
6everything stored therein is lost.
7
8tmpfs puts everything into the kernel internal caches and grows and
9shrinks to accommodate the files it contains and is able to swap
10unneeded pages out to swap space. It has maximum size limits which can
11be adjusted on the fly via 'mount -o remount ...'
12
13If you compare it to ramfs (which was the template to create tmpfs)
14you gain swapping and limit checking. Another similar thing is the RAM
15disk (/dev/ram*), which simulates a fixed size hard disk in physical
16RAM, where you have to create an ordinary filesystem on top. Ramdisks
17cannot swap and you do not have the possibility to resize them.
18
19Since tmpfs lives completely in the page cache and on swap, all tmpfs
20pages currently in memory will show up as cached. It will not show up
21as shared or something like that. Further on you can check the actual
22RAM+swap use of a tmpfs instance with df(1) and du(1).
23
24
25tmpfs has the following uses:
26
271) 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
352) 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
493) 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
544) And probably a lot more I do not know about :-)
55
56
57tmpfs has three mount options for sizing:
58
59size: 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.
63nr_blocks: The same as size, but in blocks of PAGE_CACHE_SIZE.
64nr_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
69These parameters accept a suffix k, m or g for kilo, mega and giga and
70can be changed on remount. The size parameter also accepts a suffix %
71to limit this tmpfs instance to that percentage of your physical RAM:
72the default, when neither size nor nr_blocks is specified, is size=50%
73
74If both nr_blocks (or size) and nr_inodes are set to 0, neither blocks
75nor inodes will be limited in that instance. It is generally unwise to
76mount with such options, since it allows any user with write access to
77use up all the memory on the machine; but enhances the scalability of
78that instance in a system with many cpus making intensive use of it.
79
80
81To specify the initial root directory you can use the following mount
82options:
83
84mode: The permissions as an octal number
85uid: The user id
86gid: The group id
87
88These options do not have any effect on remount. You can change these
89parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
90
91
92So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
93will give you tmpfs instance on /mytmpfs which can allocate 10GB
94RAM/SWAP in 10240 inodes and it is only accessible by root.
95
96
97Author:
98 Christoph Rohland <cr@sap.com>, 1.12.01
99Updated:
100 Hugh Dickins <hugh@veritas.com>, 01 September 2004