Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
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 <email@example.com>, 1.12.01 145 Updated: 146 Hugh Dickins, 4 June 2007 147 Updated: 148 KOSAKI Motohiro, 16 Mar 2010