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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 | ||
0edd73b3 HD |
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 | |
1da177e4 LT |
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 | To specify the initial root directory you can use the following mount | |
82 | options: | |
83 | ||
84 | mode: The permissions as an octal number | |
85 | uid: The user id | |
86 | gid: The group id | |
87 | ||
88 | These options do not have any effect on remount. You can change these | |
89 | parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem. | |
90 | ||
91 | ||
92 | So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs' | |
93 | will give you tmpfs instance on /mytmpfs which can allocate 10GB | |
94 | RAM/SWAP in 10240 inodes and it is only accessible by root. | |
95 | ||
96 | ||
97 | Author: | |
98 | Christoph Rohland <cr@sap.com>, 1.12.01 | |
99 | Updated: | |
0edd73b3 | 100 | Hugh Dickins <hugh@veritas.com>, 13 March 2005 |