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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. |
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 |
149 |
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