linux /dev/shm的用途

1.linux下的/dev/shm是什么？

/dev/shm/是linux下一个目录，/dev/shm目录不在磁盘上，而是在内存里，所以使用linux /dev/shm/的效率很是高，直接写进内存。

咱们能够经过如下两个脚原本验证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]#

能够看出，在对一个将近1g为文件的复制，拷到磁盘上与拷到/dev/shm下仍是有很大差距的。

tmpfs有如下特色：
1.tmpfs 是一个文件系统，而不是块设备；您只是安装它，它就可使用了。
2.动态文件系统的大小。
3.tmpfs 的另外一个主要的好处是它闪电般的速度。由于典型的 tmpfs 文件系统会彻底驻留在 RAM 中，读写几乎能够是瞬间的。
4.tmpfs 数据在从新启动以后不会保留，由于虚拟内存本质上就是易失的。因此有必要作一些脚本作诸如加载、绑定的操做。

2.linux /dev/shm 默认容量

linux下/dev/shm的容量默认最大为内存的一半大小，使用df -h命令能够看到。但它并不会真正的占用这块内存，若是/dev/shm/下没有任何文件，它占用的内存实际上就是0字节；若是它最大为1G，里头放有100M文件，那剩余的900M仍然可为其它应用程序所使用，但它所占用的100M内存，是毫不会被系统回收从新划分的，不然谁还敢往里头存文件呢？

经过df -h查看linux /dev/shm的大小

[root@db1 shm]# df -h /dev/shm
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 1.5G     0  1.5G   0% /dev/shm

3.linux /dev/shm 容量(大小)调整

linux /dev/shm容量(大小)是能够调整，在有些状况下(如oracle数据库)默认的最大一半内存不够用，而且默认的inode数量很低通常都要调高些，这时能够用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，将最大容量调到1.5G，而且inode数量调到1000000，这意味着大体可存入最多一百万个小文件

经过/etc/fstab文件来修改/dev/shm的容量(增长size选项便可),修改后，从新挂载便可：

[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

 

附：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