linux下的/dev/shm是什么？

/dev/shm/是linux下一個目錄，/dev/shm目錄不在磁盤上，而是在內存里，因此使用linux /dev/shm/的效率非常高，直接寫進內存。

我們可以通過以下兩個腳本來驗證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，在對一個將近1g為文件的復制，拷到磁盤上與拷到/dev/shm下還是有很大差距的。

tmpfs有以下特點：

1.tmpfs 是一個文件系統，而不是塊設備；您只是安裝它，它就可以使用了。

2.動態文件系統的大小。

3.tmpfs 的另一個主要的好處是它閃電般的速度。因為典型的 tmpfs 文件系統會完全駐留在 RAM 中，讀寫幾乎可以是瞬間的。

4.tmpfs 數據在重新啟動之后不會保留，因為虛擬內存本質上就是易失的。所以有必要做一些腳本做諸如加載、綁定的操作。

2.linux /dev/shm 默認容量

linux下/dev/shm的容量默認最大為內存的一半大小，使用df -h命令可以看到。但它并不會真正的占用這塊內存，如果/dev/shm/下沒有任何文件，它占用的內存實際上就是0字節；如果它最大為1G，里頭放有 100M文件，那剩余的900M仍然可為其它應用程序所使用，但它所占用的100M內存，是絕不會被系統回收重新劃分的，否則誰還敢往里頭存文件呢？

通過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)調整

linux /dev/shm容量(大小)是可以調整，在有些情況下(如oracle數據庫)默認的最大一半內存不夠用，并且默認的inode數量很低一般都要調高些，這時可以用mount命令來管理它。

mount -o size=1500M -o nr_inodes=1000000 -o noatime,nodiratime -o remount /dev/shm

在2G的機器上，將最大容量調到1.5G，并且inode數量調到1000000，這意味著大致可存入最多一百萬個小文件

通過/etc/fstab文件來修改/dev/shm的容量(增加size選項即可),修改后，重新掛載即可：

[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

附：tmpfs文檔

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

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