| Tmpfs is a file system which keeps all files in virtual memory. |
| |
| |
| Everything in tmpfs is temporary in the sense that no files will be |
| created on your hard drive. If you unmount a tmpfs instance, |
| everything stored therein is lost. |
| |
| tmpfs puts everything into the kernel internal caches and grows and |
| shrinks to accommodate the files it contains and is able to swap |
| unneeded pages out to swap space. It has maximum size limits which can |
| be adjusted on the fly via 'mount -o remount ...' |
| |
| If you compare it to ramfs (which was the template to create tmpfs) |
| you gain swapping and limit checking. Another similar thing is the RAM |
| disk (/dev/ram*), which simulates a fixed size hard disk in physical |
| RAM, where you have to create an ordinary filesystem on top. Ramdisks |
| cannot swap and you do not have the possibility to resize them. |
| |
| Since tmpfs lives completely in the page cache and on swap, all tmpfs |
| pages currently in memory will show up as cached. It will not show up |
| as shared or something like that. Further on you can check the actual |
| RAM+swap use of a tmpfs instance with df(1) and du(1). |
| |
| |
| tmpfs has the following uses: |
| |
| 1) There is always a kernel internal mount which you will not see at |
| all. This is used for shared anonymous mappings and SYSV shared |
| memory. |
| |
| This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not |
| set, the user visible part of tmpfs is not build. But the internal |
| mechanisms are always present. |
| |
| 2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for |
| POSIX shared memory (shm_open, shm_unlink). Adding the following |
| line to /etc/fstab should take care of this: |
| |
| tmpfs /dev/shm tmpfs defaults 0 0 |
| |
| Remember to create the directory that you intend to mount tmpfs on |
| if necessary (/dev/shm is automagically created if you use devfs). |
| |
| This mount is _not_ needed for SYSV shared memory. The internal |
| mount is used for that. (In the 2.3 kernel versions it was |
| necessary to mount the predecessor of tmpfs (shm fs) to use SYSV |
| shared memory) |
| |
| 3) Some people (including me) find it very convenient to mount it |
| e.g. on /tmp and /var/tmp and have a big swap partition. And now |
| loop mounts of tmpfs files do work, so mkinitrd shipped by most |
| distributions should succeed with a tmpfs /tmp. |
| |
| 4) And probably a lot more I do not know about :-) |
| |
| |
| tmpfs has three mount options for sizing: |
| |
| size: The limit of allocated bytes for this tmpfs instance. The |
| default is half of your physical RAM without swap. If you |
| oversize your tmpfs instances the machine will deadlock |
| since the OOM handler will not be able to free that memory. |
| nr_blocks: The same as size, but in blocks of PAGE_CACHE_SIZE. |
| nr_inodes: The maximum number of inodes for this instance. The default |
| is half of the number of your physical RAM pages, or (on a |
| a machine with highmem) the number of lowmem RAM pages, |
| whichever is the lower. |
| |
| These parameters accept a suffix k, m or g for kilo, mega and giga and |
| can be changed on remount. The size parameter also accepts a suffix % |
| to limit this tmpfs instance to that percentage of your physical RAM: |
| the default, when neither size nor nr_blocks is specified, is size=50% |
| |
| If nr_blocks=0 (or size=0), blocks will not be limited in that instance; |
| if nr_inodes=0, inodes will not be limited. It is generally unwise to |
| mount with such options, since it allows any user with write access to |
| use up all the memory on the machine; but enhances the scalability of |
| that instance in a system with many cpus making intensive use of it. |
| |
| |
| tmpfs has a mount option to set the NUMA memory allocation policy for |
| all files in that instance (if CONFIG_NUMA is enabled) - which can be |
| adjusted on the fly via 'mount -o remount ...' |
| |
| mpol=default prefers to allocate memory from the local node |
| mpol=prefer:Node prefers to allocate memory from the given Node |
| mpol=bind:NodeList allocates memory only from nodes in NodeList |
| mpol=interleave prefers to allocate from each node in turn |
| mpol=interleave:NodeList allocates from each node of NodeList in turn |
| |
| NodeList format is a comma-separated list of decimal numbers and ranges, |
| a range being two hyphen-separated decimal numbers, the smallest and |
| largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15 |
| |
| |
| To specify the initial root directory you can use the following mount |
| options: |
| |
| mode: The permissions as an octal number |
| uid: The user id |
| gid: The group id |
| |
| These options do not have any effect on remount. You can change these |
| parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem. |
| |
| |
| So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs' |
| will give you tmpfs instance on /mytmpfs which can allocate 10GB |
| RAM/SWAP in 10240 inodes and it is only accessible by root. |
| |
| |
| Author: |
| Christoph Rohland <cr@sap.com>, 1.12.01 |
| Updated: |
| Hugh Dickins <hugh@veritas.com>, 19 February 2006 |