Linux 虚拟内存的使用

使用阿里云服务器经常遇到内存不足造成服务器挂掉的问题,没办法改变物理内存太小，那就弄虚拟内存吧。

Linux系统实现虚拟内存有两种方法：交换分区（swap分区）和交换文件

交换文件

• 查看内存 (-m显示单位为MB,-g显示单位为GB)

[root@caoxl /]# free -m
              total        used        free      shared  buff/cache   available
Mem:           3789         111        3079           9         599        3435
Swap:             0           0           0

[root@caoxl /]# free -g
              total        used        free      shared  buff/cache   available
Mem:              3           0           3           0           0           3
Swap:             0           0           0

• 创建一个文件

[root@caoxl /]# touch swap
[root@caoxl /]# ls
bin   data  download  home  lib64       media  opt   root  sbin  swap  tmp  var
boot  dev   etc       lib   lost+found  mnt    proc  run   srv   sys   usr

• 使用dd命令,创建大小为2G的文件swap

[root@caoxl /]# dd if=/dev/zero of=/swap bs=1M count=2048
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 11.7135 s, 183 MB/s

• if - input_file 输入文件
• of - output_file 输出文件
• bs - block_size 块大小
• count 表示计数

• 格式化交换文件

[root@caoxl /]# mkswap swap
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=f77dd6aa-0625-47e9-9eb9-bf26050a8508

• 启用交换文件

[root@caoxl /]# swapon swap
swapon: /swap: insecure permissions 0644, 0600 suggested.

• 开机自动加载虚拟内存,在/etc/fstab文件中加入如下命令

[root@caoxl etc]# vim fstab 
/swap swap swap defults 0 0

• 重启后生效reboot

[root@caoxl /]# free -m
              total        used        free      shared  buff/cache   available
Mem:           3789         110        1309           4        2369        3424
Swap:          2047           0        2047

删除交换分区和交换文件

• 先删除/etc/fstab文件中添加的交换文件行

• 停用交换文件

[root@caoxl /]# swapoff /swap

• 删除交换文件

[root@caoxl /]# rm -rf ./swap 

[root@caoxl /]# free -m
              total        used        free      shared  buff/cache   available
Mem:           3789         109        3416           4         264        3454
Swap:             0           0           0

交换分区

• 查看硬盘信息

[root@caoxl ~]# fdisk -l

Disk /dev/vda: 42.9 GB, 42949672960 bytes, 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x0008d73a

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *        2048    83884031    41940992   83  Linux

• 查看挂载信息

[root@caoxl ~]# df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
/dev/vda1      ext4       40G   13G   26G  33% /
devtmpfs       devtmpfs  1.9G     0  1.9G   0% /dev
tmpfs          tmpfs     1.9G     0  1.9G   0% /dev/shm
tmpfs          tmpfs     1.9G  484K  1.9G   1% /run
tmpfs          tmpfs     1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs          tmpfs     379M     0  379M   0% /run/user/0

• 以root身份创建分区 hdb1 fdisk /dev/vda1 然后 w 保存退出

[root@caoxl /]# sudo fdisk /dev/vda1
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0xc5d30913.

Command (m for help): n                         // 新增分区
Partition type:
   p   primary (0 primary, 0 extended, 4 free)  // 主分区
   e   extended                                 // 扩展分区
Select (default p): e                           // 选择要创建的分区类型
Partition number (1-4, default 1): 1            // 分区编号 
First sector (2048-83881983, default 2048):     // 柱面起始值,直接回车默认
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-83881983, default 83881983): +1G
Partition 1 of type Extended and of size 1 GiB is set

Command (m for help): w                         // 保存分区表
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 22: Invalid argument.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.

• 查看分区

Command (m for help): p

Disk /dev/vda1: 42.9 GB, 42947575808 bytes, 83881984 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0xc5d30913

     Device Boot      Start         End      Blocks   Id  System
/dev/vda1p1            2048     2099199     1048576    5  Extended
/dev/vda1p2         2099200     3147775      524288   83  Linux

• 设置交换分区

[root@caoxl /]# mkswap /dev/vda1

• 启用交换分区

[root@caoxl /]# swapon /dev/vda1

• 开机自动加载虚拟内存,在/etc/fstab文件中加入如下命令

/dev/vda1 swap swap defults 0 0