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最新评论
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massjcy:
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如何将ubuntu文件夹中文名改为英文 -
skypiea:
谢谢。。。
终于解决了。。。
Oracle 10.2.0.4(5)EM不能启动的解决方案(Patch 8350262) -
qwe_rt:
引用vi /etc/sysconfig/network 请问 ...
Linux操作系统下配置静态IP上网 -
liuqiang:
sudo killall -9 apache2
ps 和 kill 命令详解 -
dazuiba:
引用*绝杀 kill -9 PID 当使用此命令时,一定要通过 ...
ps 和 kill 命令详解
linux /dev/shm的用途
- 博客分类:
- linux学习
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,因此使用linux /dev/shm/的效率非常高,直接写进内存。
我们可以通过以下两个脚本来验证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为文件的复制,拷到磁盘上与拷到/dev/shm下还是有很大差距的。
tmpfs有以下特点:
1.tmpfs 是一个文件系统,而不是块设备;您只是安装它,它就可以使用了。
2.动态文件系统的大小。
3.tmpfs 的另一个主要的好处是它闪电般的速度。因为典型的 tmpfs 文件系统会完全驻留在 RAM 中,读写几乎可以是瞬间的。
4.tmpfs 数据在重新启动之后不会保留,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。
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命令可以看到。但它并不会真正的占用这块内存,如果/dev/shm/下没有任何文件,它占用的内存实际上就是0字节;如果它最大为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,是绝不会被系统回收重新划分的,否则谁还敢往里头存文件呢?
通过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)是可以调整,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,将最大容量调到1.5G,并且inode数量调到1000000,这意味着大致可存入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
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
发表评论
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2012-08-31 17:52 1696自己写的通用的sqlldr导入脚本,配置在crontab 里面 ... -
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linux下删除文件后没有释放空间(转)
2012-08-08 11:07 1924问题 文件被删除后,空间没有被释放 原因 在Linux或者 ... -
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configure/make/make install的作用
2011-08-12 01:10 859这些都是典型的使用GNU的AUTOCONF和AUTOMAKE产 ... -
执行脚本出现bin/bash: bad interpreter: No such file or directory
2011-07-27 16:50 722错误原因之一很有可能是你的脚本文件是DOS格式的, 即每一行的 ... -
Ubuntu下Oracle的自动启动
2011-07-06 23:38 2442一、使用root用户修改/etc/oratab 文件: $ ... -
如何将ubuntu文件夹中文名改为英文
2011-03-07 02:10 8531ubuntu对中文的支持越来越好,这自然是好事。不过在终端下输 ... -
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2011-03-07 00:53 922下面是Ubuntu 10.04下的搭 ... -
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2011-03-07 00:43 11611.准备环境首先得安装好ubuntu-10.04-server ... -
ubuntu安装SSH,支持开启sftp服务
2011-03-06 23:55 2695ubuntu安装SSH 安装 SSH(Secure Shel ... -
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2011-03-05 23:12 1813解决方法: 一. Ubuntu默认的中文字符编 ... -
linux使用sar命令进行系统监视优化
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Linux启动过程简介
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