使用磁盘为Linux添加swap

一．SWAP 说明
 
1、SWAP 概述
　　当系统的物理内存不够用的时候，就需要将物理内存中的一部分空间释放出来，以供当前运行的程序使用。那些被释放的空间可能来自一些很长时间没有什么操作的程序，这些被释放的空间被临时保存到Swap空间中，等到那些程序要运行时，再从Swap中恢复保存的数据到内存中。这样，系统总是在物理内存不够时，才进行Swap交换。
　　以上是SWAP 交换分区的作用。 实际上，我们更关注的应该是SWAP分区的大小问题。 设置多大才是最优的。一般来说可以按照如下规则设置swap大小：
　　　　

　　　　4G以内的物理内存，SWAP 设置为内存的2倍。
　　　　4-8G的物理内存，SWAP 等于内存大小。
　　　　8-64G 的物理内存，SWAP 设置为8G。
　　　　64-256G物理内存，SWAP 设置为16G。
　　

　　实际上，系统中交换分区的大小并不取决于物理内存的量，而是取决于系统中内存的负荷，所以在安装系统时要根据具体的业务来设置SWAP的值。

2、系统在什么情况下才会使用SWAP？
　　实际上，并不是等所有的物理内存都消耗完毕之后，才去使用swap的空间，什么时候使用是由swappiness 参数值控制。

[root@rhce ~]# cat /proc/sys/vm/swappiness

　　该值默认值是60。swappiness=0的时候表示最大限度使用物理内存，然后才是 swap空间，swappiness＝100的时候表示积极的使用swap分区，并且把内存上的数据及时的搬运到swap空间里面。现在服务器的内存动不动就是上百G，所以我们可以把这个参数值设置的低一些，让操作系统尽可能的使用物理内存，降低系统对swap的使用，从而提高系统的性能。

二．使用磁盘为Linux添加swap

　　查看磁盘及分区信息

[root@ip---- ~]# fdisk -l

Disk /dev/xvda: 193.3 GB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes
Disk label type: dos
Disk identifier: 0x000da64d

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *                   Linux

Disk /dev/mapper/docker-:--pool: 107.4 GB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes

Disk /dev/xvdf:  MB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes

　　添加新的分区

[root@ip---- ~]# fdisk /dev/xvdf
Welcome to fdisk (util-linux 2.23.).

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 0x8fb93abe.

Command (m for help): m
Command action
   a   toggle a bootable flag
   b   edit bsd disklabel
   c   toggle the dos compatibility flag
   d   delete a partition
   g   create a new empty GPT partition table
   G   create an IRIX (SGI) partition table
   l   list known partition types
   m   print this menu
   n   add a new partition
   o   create a new empty DOS partition table
   p   print the partition table
   q   quit without saving changes
   s   create a new empty Sun disklabel
   t   change a partition's system id
   u   change display/entry units
   v   verify the partition table
   w   write table to disk and exit
   x   extra functionality (experts only)

Command (m for help): n
Partition type:
   p   primary ( primary,  extended,  free)
   e   extended
Select (default p): p
Partition number (-, default ): 
First sector (-, default ):  （直接回车）
Using default value
Last sector, +sectors or +size{K,M,G} (-, default ):  （直接回车）
Using default value
Partition  of type Linux and of size  GiB is set

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

　　查看磁盘及分区信息

[root@ip---- ~]# fdisk -l

Disk /dev/xvda: 193.3 GB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes
Disk label type: dos
Disk identifier: 0x000da64d

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *                   Linux

Disk /dev/mapper/docker-:--pool: 107.4 GB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes

Disk /dev/xvdf:  MB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes
Disk label type: dos
Disk identifier: 0x8fb93abe

    Device Boot      Start         End      Blocks   Id  System
/dev/xvdf1                           Linux

　　可以看到/dev/xvdf1是新创建的分区，注意这里fdisk ID的编号。 这里显示的83，它代表这个分区是ext的分区，这个是不能用来做swap的。我们继续更改

[root@ip---- ~]# fdisk /dev/xvdf
Welcome to fdisk (util-linux 2.23.).

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

Command (m for help): l  （list known partition types）

   Empty             NEC DOS           Minix / old Lin bf  Solaris
   FAT12             Hidden NTFS Win   Linux swap / So c1  DRDOS/sec (FAT-
   XENIX root        Plan             Linux           c4  DRDOS/sec (FAT-
   XENIX usr       3c  PartitionMagic    OS/ hidden C:  c6  DRDOS/sec (FAT-
   FAT16 <32M        Venix        Linux extended  c7  Syrinx
   Extended          PPC PReP Boot     NTFS volume set da  Non-FS data
   FAT16             SFS               NTFS volume set db  CP/M / CTOS / .
   HPFS/NTFS/exFAT 4d  QNX4.x            Linux plaintext de  Dell Utility
   AIX             4e  QNX4.x 2nd part 8e  Linux LVM       df  BootIt
   AIX bootable    4f  QNX4.x 3rd part   Amoeba          e1  DOS access
 a  OS/ Boot Manag   OnTrack DM        Amoeba BBT      e3  DOS R/O
 b  W95 FAT32         OnTrack DM6 Aux 9f  BSD/OS          e4  SpeedStor
 c  W95 FAT32 (LBA)   CP/M            a0  IBM Thinkpad hi eb  BeOS fs
 e  W95 FAT16 (LBA)   OnTrack DM6 Aux a5  FreeBSD         ee  GPT
 f  W95 Ext'd (LBA) 54  OnTrackDM6      a6  OpenBSD         ef  EFI (FAT-12/16/
  OPUS              EZ-Drive        a7  NeXTSTEP        f0  Linux/PA-RISC b
  Hidden FAT12      Golden Bow      a8  Darwin UFS      f1  SpeedStor
  Compaq diagnost 5c  Priam Edisk     a9  NetBSD          f4  SpeedStor
  Hidden FAT16 <   SpeedStor       ab  Darwin boot     f2  DOS secondary
  Hidden FAT16      GNU HURD or Sys af  HFS / HFS+      fb  VMware VMFS
  Hidden HPFS/NTF   Novell Netware  b7  BSDI fs         fc  VMware VMKCORE
  AST SmartSleep    Novell Netware  b8  BSDI swap       fd  Linux raid auto
1b  Hidden W95 FAT3   DiskSecure Mult bb  Boot Wizard hid fe  LANstep
1c  Hidden W95 FAT3   PC/IX           be  Solaris boot    ff  BBT
1e  Hidden W95 FAT1   Old Minix      

Command (m for help): m
Command action
   a   toggle a bootable flag
   b   edit bsd disklabel
   c   toggle the dos compatibility flag
   d   delete a partition
   g   create a new empty GPT partition table
   G   create an IRIX (SGI) partition table
   l   list known partition types
   m   print this menu
   n   add a new partition
   o   create a new empty DOS partition table
   p   print the partition table
   q   quit without saving changes
   s   create a new empty Sun disklabel
   t   change a partition's system id
   u   change display/entry units
   v   verify the partition table
   w   write table to disk and exit
   x   extra functionality (experts only)

Command (m for help): t
Selected partition 1
Hex code (type L to list all codes): 82
Changed type of partition 'Linux' to 'Linux swap / Solaris'

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

　　查看磁盘及分区信息

[root@ip---- ~]# fdisk -l

Disk /dev/xvda: 193.3 GB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes
Disk label type: dos
Disk identifier: 0x000da64d

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *                   Linux

Disk /dev/mapper/docker-:--pool: 107.4 GB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes

Disk /dev/xvdf:  MB,  bytes,  sectors
Units = sectors of  *  =  bytes
Sector size (logical/physical):  bytes /  bytes
I/O size (minimum/optimal):  bytes /  bytes
Disk label type: dos
Disk identifier: 0x8fb93abe

    Device Boot      Start         End      Blocks   Id  System
/dev/xvdf1                           Linux swap / Solaris

　　格式化swap分区：

[root@rhce /]# mkswap /dev/xvdf1
Setting up swapspace version , size =  KiB
no label, UUID=39c73cf6--4a78-83ec-35089f0eebcb

[root@ip-192-168-31-161 ~]# blkid

/dev/xvda1: LABEL="centos7_root" UUID="179c37d9-9ba3-47f2-ab50-cf76dec5ef8e" TYPE="xfs" 

/dev/loop0: UUID="0c7e9760-c4df-4d5b-b0b6-b2f4487e757b" TYPE="ext4"
/dev/mapper/docker-202:1-385876097-pool: UUID="0c7e9760-c4df-4d5b-b0b6-b2f4487e757b" TYPE="ext4"
/dev/xvdf1: UUID="39c73cf6-1848-4a78-83ec-35089f0eebcb" TYPE="swap"

　　手工的启动swap

[root@ip-192-168-31-161 ~]# swapon /dev/xvdf1

[root@ip---- ~]# swapon -s

　　将/dev/xvdf1添加/etc/fstab文件，系统启动自动加载swap

[root@ip---- ~]# vi /etc/fstab
#
# /etc/fstab
# Created by anaconda on Tue Jul   ::
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(), findfs(), mount() and/or blkid() for more info
#
LABEL=centos7_root /                       xfs     defaults
UUID=39c73cf6--4a78-83ec-35089f0eebcb  swap     swap    defaults