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