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发表于 2006-7-21 00:31:26
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引自内核源文件 Documentation/filesystems/tmpfs.txt
- 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
- Note that trying to mount a tmpfs with an mpol option will fail if the
- running kernel does not support NUMA; and will fail if its nodelist
- specifies a node >= MAX_NUMNODES. If your system relies on that tmpfs
- being mounted, but from time to time runs a kernel built without NUMA
- capability (perhaps a safe recovery kernel), or configured to support
- fewer nodes, then it is advisable to omit the mpol option from automatic
- mount options. It can be added later, when the tmpfs is already mounted
- on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
- 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
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