Linux Add a Swap File

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Procedure To Add a Swap File Under Linux

You need to use the dd command to create swap file. The mkswap command is used to set up a Linux swap area on a device or in a file.

Step #1: Login as the Root User

Open a terminal window (select Applications > Accessories > Terminal) or login to remote server using the ssh client. Switch to the root user by typing su - (or sudo -s) and entering the root password, when prompted:
$ su -
OR
$ sudo -s

Step #2: Create Storage File

Type the following command to create 512MB swap file (1024 * 512MB = 524288 block size):
# dd if=/dev/zero of=/swapfile1 bs=1024 count=524288
Sample outputs:

524288+0 records in524288+0 records out536870912 bytes (537 MB) copied, 3.23347 s, 166 MB/s

Where,

1. if=/dev/zero : Read from /dev/zero file. /dev/zero is a special file in that provides as many null characters to build storage file called /swapfile1.
2. of=/swapfile1 : Read from /dev/zero write storage file to /swapfile1.
3. bs=1024 : Read and write 1024 BYTES bytes at a time.
4. count=524288 : Copy only 523288 BLOCKS input blocks.

Step #3: Set Up a Linux Swap Area

Type the following command to set up a Linux swap area in a file:
# mkswap /swapfile1
Sample outputs:

Setting up swapspace version 1, size = 524284 KiBno label, UUID=e1fe2038-6554-4d1a-a108-037acd4e02e2

Setup correct file permission for security reasons, enter:
# chown root:root /swapfile1
# chmod 0600 /swapfile1
A world-readable swap file is a huge local vulnerability. The above command make sure only root user can read/write to the file. Finally, activate /swapfile1 swap space immediately, enter:
# swapon /swapfile1

Update /etc/fstab file

To activate /swapfile1 after Linux system reboot, add entry to /etc/fstab file. Open this file using a text editor such as vi:
# vi /etc/fstab
Append the following line:
/swapfile1 swap swap defaults 0 0
Save and close the file. Next time Linux comes up after reboot, it enables the new swap file for you automatically.

How do I Verify Swap is Activated or Not?

Simply use the free command:
$ free -m

             total       used       free     shared    buffers     cachedMem:          1876       1798         77          0        119       1440-/+ buffers/cache:        237       1638Swap:         4607          0       4607

How can I display swap usage summary on Linux?

Type the following swapon command:
# swapon -s
Sample outputs:

FilenameTypeSizeUsedPriority/dev/sda6                               partition419429600/swapfile1                              file5242800-1

Another option is to view /proc/meminfo file:
$ less /proc/meminfo
$ grep -i --color swap /proc/meminfo
Sample outputs:

SwapCached:        30748 kBSwapTotal:       6291448 kBSwapFree:        6154008 kB

You can also use top command, atop command, and/or htop command to display information about swap usage:
# top
# atop
# htop
Sample outputs from a database server running on a CentOS Linux server:

  1  [||                        3.9%]     Tasks: 171, 106 thr; 1 running  2  [                          0.0%]     Load average: 0.06 0.12 0.09  3  [||                        2.0%]     Uptime: 22 days, 07:07:28  4  [                          0.0%]  5  [                          0.0%]  6  [                          0.0%]  7  [                          0.0%]  8  [                          0.0%]  Mem[||||||||||||||||||2112/11909MB]  Swp[|                   134/6143MB]  PID USER      PRI  NI  VIRT   RES   SHR S CPU% MEM%   TIME+  Command 8523 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:00.00 /usr/libexec/mysq 8524 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:02.74 /usr/libexec/mysq 8525 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:16.17 /usr/libexec/mysq 8526 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:06.33 /usr/libexec/mysq 8528 mysql      20   0 4545M  414M  4816 S  0.0  3.5  4:02.14 /usr/libexec/mysq 8529 mysql      20   0 4545M  414M  4816 S  0.0  3.5  5:22.00 /usr/libexec/mysq 8530 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:04.63 /usr/libexec/mysq 8531 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:50.95 /usr/libexec/mysq 8532 mysql      20   0 4545M  414M  4816 S  0.0  3.5  0:08.10 /usr/libexec/mysq 9359 mysql      20   0 4545M  414M  4816 S  0.0  3.5 22:53.28 /usr/libexec/mysq

How can I disable devices and files for paging and swapping on Linux?

You need to use the swapoff command:
# swapoff /swapfile1
# swapon -s

How do I set swappiness on a Linux server?

The syntax is:
# sysctl vm.swappiness=VALUE
# sysctl vm.swappiness=20
OR
# echo VALUE > /proc/sys/vm/swappiness
# echo 30 > /proc/sys/vm/swappiness

The value in /proc/sys/vm/swappiness file controls how aggressively the kernel will swap memory pages. Higher values increase agressiveness, lower values descrease aggressiveness. The default value is 60. To make changes permanent add the following line to /etc/sysctl.conf:

 echo ‘vm.swappiness=30‘ >> /etc/sysctl.conf 

Linux Add a Swap File