/dev/shm是Linux下的一个目录，/dev/shm目录不在磁盘上，而是在内存中，因此使用Linux /dev/shm的效率非常高，直接写进内存
可以通过两个脚本验证Linux /dev/shm的性能
[root@db1 oracle]# ls -l linux_11gR2_grid.zip
-rw-r--r-- 1 oracle dba 980831749 Jul 11 20:18 linux_11gR2_grid.zip
[root@db1 oracle]# cat mycp.sh
#!/bin/sh
echo `date`
cp linux_11gR2_grid.zip ..
echo `date`
[root@db1 oracle]# ./mycp.sh
Fri Jul 15 18:44:17 CST 2011
Fri Jul 15 18:44:29 CST 2011
[root@db1 shm]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/rootvg-lv01
97G 9.2G 83G 10% /
/dev/sda1 99M 15M 80M 16% /boot
tmpfs 2.0G 0 2.0G 0% /dev/shm
[root@db1 oracle]# cat mycp1.sh
#!/bin/sh
echo `date`
cp linux_11gR2_grid.zip /dev/shm
echo `date`
[root@db1 oracle]# ./mycp1.sh
Fri Jul 15 18:44:29 CST 2011
Fri Jul 15 18:44:30 CST 2011
[root@db1 oracle]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/rootvg-lv01
97G 9.2G 83G 10% /
/dev/sda1 99M 15M 80M 16% /boot
tmpfs 2.0G 937M 1.1G 46% /dev/shm
[root@db1 oracle]#
可以看出在对一个将近1G文件的复制，拷到磁盘上与拷到shm下还是差距很大的
tmpfs有以下特点：
1 tmpfs是一个文件系统，而不是块设备；只要安装，就可以使用
2 动态文件系统的大小
3 tmpfs的另一个好处是闪电般的速度，因为典型的tmpfs文件系统会完成驻留在RAM中，读写几乎是瞬间的
4 tmpfs数据在重启之后不会保留，因为虚拟内存本质上就是易失的，所以有必要做一些脚本做诸如加载，绑定的操作
Linux下/dev/shm的容量默认最大为内存的一半大小，使用df -h命令可以看到，但它不会真正的占有这块内存，如果/dev/shm下没有任何文件，它占有的内存实际是0字节，如果它最大是1G，里面放100M文件，那剩余的900M任然可以为其他程序使用，但它所占的100M内存，是绝对不会被系统回收重新划分的，否则没人敢在这里面存文件
[root@db1 shm]# df -h /dev/shm Filesystem Size Used Avail Use% Mounted on tmpfs 1.5G 0 1.5G 0% /dev/shm
Linux下/dev/shm容量大小是可以调整的，在有些情况下如Oracle数据库默认的最大一半内存不够用，并且默认的inode数量很低一般都要调高些，这时可以用mount命令来管理它
mount -o size=1500M -o nr_inodes=1000000 -o noatime,nodiratime -o remount /dev/shm
在2G的机器上，调整到1.5G，并且inode调整到100w，这意味着大致可以存放最多一百万个小文件
通过/etc/fstab文件来修改/dev/shm的容量（增加size选项即可）修改后，重新挂载即可
[root@db1 shm]# grep tmpfs /etc/fstab
tmpfs /dev/shm tmpfs defaults,size=2G 0 0
[root@db1 /]# umount /dev/shm
[root@db1 /]# mount /dev/shm
[root@db1 /]# df -h /dev/shm
Filesystem Size Used Avail Use% Mounted on
tmpfs 2.0G 0 2.0G 0% /dev/shm
[root@db1 /]# # mount -o remount /dev/shm
[root@db1 /]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/rootvg-lv01
97G 9.2G 83G 10% /
/dev/sda1 99M 15M 80M 16% /boot
tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs文档
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. But be aware: loop mounts of tmpfs files do not work due to the internal design. So mkinitrd shipped by most distributions will fail with a tmpfs /tmp.
4) And probably a lot more I do not know about 
tmpfs has a couple of mount options:
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 PAGECACHE_SIZE.
nr_inodes: The maximum number of inodes for this instance. The default is half of the number of your physical RAM pages.
These parameters accept a suffix k, m or g for kilo, mega and giga and can be changed on remount.
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.
TODOs:
1) give the size option a percent semantic: If you give a mount option size=50% the tmpfs instance should be able to grow to 50 percent of RAM + swap. So the instance should adapt automatically if you add or remove swap space.
2) loop mounts: This is difficult since loop.c relies on the readpage operation. This operation gets a page from the caller to be filled with the content of the file at that position. But tmpfs always has the page and thus cannot copy the content to the given page. So it cannot provide this operation. The VM had to be changed seriously to achieve this.
3) Show the number of tmpfs RAM pages. (As shared?)
Author:
Christoph Rohland , 1.12.01