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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.
-
- 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
- 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 use the process allocation policy
- (see set_mempolicy(2))
-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
-mpol=local prefers to allocate memory from the local node
-
-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
-
-A memory policy with a valid NodeList will be saved, as specified, for
-use at file creation time. When a task allocates a file in the file
-system, the mount option memory policy will be applied with a NodeList,
-if any, modified by the calling task's cpuset constraints
-[See Documentation/cgroups/cpusets.txt] and any optional flags, listed
-below. If the resulting NodeLists is the empty set, the effective memory
-policy for the file will revert to "default" policy.
-
-NUMA memory allocation policies have optional flags that can be used in
-conjunction with their modes. These optional flags can be specified
-when tmpfs is mounted by appending them to the mode before the NodeList.
-See Documentation/vm/numa_memory_policy.txt for a list of all available
-memory allocation policy mode flags and their effect on memory policy.
-
- =static is equivalent to MPOL_F_STATIC_NODES
- =relative is equivalent to MPOL_F_RELATIVE_NODES
-
-For example, mpol=bind=static:NodeList, is the equivalent of an
-allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
-
-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 which is not online. 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 with fewer nodes
-online, 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, 4 June 2007
-Updated:
- KOSAKI Motohiro, 16 Mar 2010