forked from luck/tmp_suning_uos_patched
1da177e4c3
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!
101 lines
4.1 KiB
Plaintext
101 lines
4.1 KiB
Plaintext
Tmpfs is a file system which keeps all files in virtual memory.
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Everything in tmpfs is temporary in the sense that no files will be
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created on your hard drive. If you unmount a tmpfs instance,
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everything stored therein is lost.
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tmpfs puts everything into the kernel internal caches and grows and
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shrinks to accommodate the files it contains and is able to swap
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unneeded pages out to swap space. It has maximum size limits which can
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be adjusted on the fly via 'mount -o remount ...'
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If you compare it to ramfs (which was the template to create tmpfs)
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you gain swapping and limit checking. Another similar thing is the RAM
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disk (/dev/ram*), which simulates a fixed size hard disk in physical
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RAM, where you have to create an ordinary filesystem on top. Ramdisks
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cannot swap and you do not have the possibility to resize them.
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Since tmpfs lives completely in the page cache and on swap, all tmpfs
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pages currently in memory will show up as cached. It will not show up
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as shared or something like that. Further on you can check the actual
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RAM+swap use of a tmpfs instance with df(1) and du(1).
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tmpfs has the following uses:
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1) There is always a kernel internal mount which you will not see at
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all. This is used for shared anonymous mappings and SYSV shared
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memory.
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This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
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set, the user visible part of tmpfs is not build. But the internal
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mechanisms are always present.
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2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
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POSIX shared memory (shm_open, shm_unlink). Adding the following
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line to /etc/fstab should take care of this:
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tmpfs /dev/shm tmpfs defaults 0 0
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Remember to create the directory that you intend to mount tmpfs on
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if necessary (/dev/shm is automagically created if you use devfs).
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This mount is _not_ needed for SYSV shared memory. The internal
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mount is used for that. (In the 2.3 kernel versions it was
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necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
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shared memory)
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3) Some people (including me) find it very convenient to mount it
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e.g. on /tmp and /var/tmp and have a big swap partition. And now
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loop mounts of tmpfs files do work, so mkinitrd shipped by most
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distributions should succeed with a tmpfs /tmp.
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4) And probably a lot more I do not know about :-)
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tmpfs has three mount options for sizing:
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size: The limit of allocated bytes for this tmpfs instance. The
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default is half of your physical RAM without swap. If you
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oversize your tmpfs instances the machine will deadlock
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since the OOM handler will not be able to free that memory.
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nr_blocks: The same as size, but in blocks of PAGE_CACHE_SIZE.
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nr_inodes: The maximum number of inodes for this instance. The default
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is half of the number of your physical RAM pages, or (on a
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a machine with highmem) the number of lowmem RAM pages,
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whichever is the lower.
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These parameters accept a suffix k, m or g for kilo, mega and giga and
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can be changed on remount. The size parameter also accepts a suffix %
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to limit this tmpfs instance to that percentage of your physical RAM:
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the default, when neither size nor nr_blocks is specified, is size=50%
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If both nr_blocks (or size) and nr_inodes are set to 0, neither blocks
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nor inodes will be limited in that instance. It is generally unwise to
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mount with such options, since it allows any user with write access to
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use up all the memory on the machine; but enhances the scalability of
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that instance in a system with many cpus making intensive use of it.
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To specify the initial root directory you can use the following mount
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options:
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mode: The permissions as an octal number
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uid: The user id
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gid: The group id
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These options do not have any effect on remount. You can change these
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parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
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So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
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will give you tmpfs instance on /mytmpfs which can allocate 10GB
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RAM/SWAP in 10240 inodes and it is only accessible by root.
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Author:
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Christoph Rohland <cr@sap.com>, 1.12.01
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Updated:
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Hugh Dickins <hugh@veritas.com>, 01 September 2004
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