linux之tmpfs

Tmpfs is a file system which keeps all files in virtual memory.Everything in tmpfs is temporary in the sense that no files will becreated 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 andshrinks to accommodate the files it contains and is able to swapunneeded pages out to swap space. It has maximum size limits which canbe 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 RAMdisk (/dev/ram*), which simulates a fixed size hard disk in physicalRAM, where you have to create an ordinary filesystem on top. Ramdiskscannot swap and you do not have the possibility to resize them. Since tmpfs lives completely in the page cache and on swap, all tmpfspages currently in memory will show up as cached. It will not show upas shared or something like that. Further on you can check the actualRAM+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/shmtmpfsdefaults0 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 andcan 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 tomount with such options, since it allows any user with write access touse up all the memory on the machine; but enhances the scalability ofthat instance in a system with many cpus making intensive use of it.tmpfs has a mount option to set the NUMA memory allocation policy forall files in that instance (if CONFIG_NUMA is enabled) - which can beadjusted 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 Nodempol=bind:NodeList       allocates memory only from nodes in NodeListmpol=interleave          prefers to allocate from each node in turnmpol=interleave:NodeList allocates from each node of NodeList in turnmpol=local prefers to allocate memory from the local nodeNodeList format is a comma-separated list of decimal numbers and ranges,a range being two hyphen-separated decimal numbers, the smallest andlargest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15A memory policy with a valid NodeList will be saved, as specified, foruse at file creation time.  When a task allocates a file in the filesystem, 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, listedbelow.  If the resulting NodeLists is the empty set, the effective memorypolicy for the file will revert to "default" policy.NUMA memory allocation policies have optional flags that can be used inconjunction with their modes.  These optional flags can be specifiedwhen tmpfs is mounted by appending them to the mode before the NodeList.See Documentation/vm/numa_memory_policy.txt for a list of all availablememory allocation policy mode flags and their effect on memory policy.=staticis equivalent toMPOL_F_STATIC_NODES=relativeis equivalent toMPOL_F_RELATIVE_NODESFor example, mpol=bind=static:NodeList, is the equivalent of anallocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.Note that trying to mount a tmpfs with an mpol option will fail if therunning kernel does not support NUMA; and will fail if its nodelistspecifies a node which is not online.  If your system relies on thattmpfs being mounted, but from time to time runs a kernel built withoutNUMA capability (perhaps a safe recovery kernel), or with fewer nodesonline, then it is advisable to omit the mpol option from automaticmount options.  It can be added later, when the tmpfs is already mountedon MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.To specify the initial root directory you can use the following mountoptions:mode:The permissions as an octal numberuid:The user id gid:The group idThese options do not have any effect on remount. You can change theseparameters 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 10GBRAM/SWAP in 10240 inodes and it is only accessible by root.Author:   Christoph Rohland <[e-mail]>, 1.12.01Updated:   Hugh Dickins, 4 June 2007Updated:   KOSAKI Motohiro, 16 Mar 2010来自：https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt