path: root/Documentation/filesystems/tmpfs.txt
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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/filesystems/tmpfs.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+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 both nr_blocks (or size) and nr_inodes are set to 0, neither blocks
+nor inodes will be limited in that instance. 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.
+To specify the initial root directory you can use the following mount
+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.
+ Christoph Rohland <cr@sap.com>, 1.12.01
+ Hugh Dickins <hugh@veritas.com>, 01 September 2004