How To Add Swap to Linux Box

In this tutorial we will see how we can add SWAP space to our Linux box and also understand the use of SWAP space.

What is Swap ?

Swap is an area on a hard drive that has been designated as a place where the operating system can temporarily store data that it can no longer hold in RAM. Basically, this gives you the ability to increase the amount of information that your server can keep in its working "memory". The space on the hard drive will be used mainly when space in RAM is no longer sufficient for data. Very important to understand is that the information written to disk will be slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a backup plan when your system's RAM is almost done and swap space will save your skin. swap So now that we know what is SWAP and why we use it lets see how we can get information about our system SWAP, how we can add more SWAP or even remove SWAP space.

1.See if you have SWAP:

We can see if the system has any configured SWAP by typing: 
[root@aodba]# swapon -s
Filename         Type            Size    Used    Priority
  • no SWAP here !!!
Also free command with option -m can tell use about our SWAP 
[root@aodba]# free -m
             total       used       free     shared    buffers     cached
Mem:           594        378        215          0         30        274
-/+ buffers/cache:         74        520
Swap:            0          0          0
  • As you can see our total swap space in the system is 0.

2. Add SWAP

Before you add SWAP you need to make sure you have the disk space on your host, df command with option -h is a good candidate for this task. 
[root@aodba]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/xvda1            7.9G  1.9G  6.0G  24% /
tmpfs                 298M     0  298M   0% /dev/shm
  • ok we got some space left.
SWAP space can be added using the dd command or fallocate command. dd           -  pre allocates a portion of the disk
  • you can use dd to write zeros to the file from a special device in Linux systems located at /dev/zero that just spits out as many zeros as requested.(pretty smart :) )
fallocate -  creates a file of a pre allocated size instantly, without actually having to write dummy contents. Create using dd:
  • the command will spit 1024 mb (zeross) into the file /SWAPfile
[root@aodba]# dd if=/dev/zero of=/SWAPfile bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 30.0351 s, 35.7 MB/s

[root@aodba]# ls -la /SWAPfile
-rw-r--r-- 1 root root 1073741824 Jul 27 05:14 /SWAPfile
  • SWAP file create
Create using the fallocate: 
[root@aodba]# fallocate -l 1G /SWAPfile

[root@aodba]# ls -la /SWAPfile
-rw-r--r-- 1 root root 1073741824 Jul 27 05:18 /SWAPfile
  • this way is instant file allocation.

3.Enabling the Swap File

At this point we have the file created and we need to hand it to our operational system for use. Before we go ahead and do this we need to make sure this file can be read only by the root user. Change file permissions: 
[root@aodba]#chmod 600 /SWAPfile

[root@aodba]# ls -la /SWAPfile
-rw------- 1 root root 1073741824 Jul 27 05:18 /SWAPfile
Setup SWAP by using the mkswap command: 
[root@aodba]# mkswap /SWAPfile
Setting up swapspace version 1, size = 1048572 KiB
no label, UUID=b8e7b669-8b26-4d70-bd60-d7678427e3dc
Make SWAP available now: 
[root@aodba]# swapon /SWAPfile
Verify that SWAP is there: 
[root@aodba]# free -m
             total       used       free     shared    buffers     cached
Mem:           594        264        329          0         12        185
-/+ buffers/cache:         66        527
Swap:         1023          0       1023
[root@aodba]# swapon -s
Filename                                Type            Size    Used    Priority
/SWAPfile                               file            1048572 0       -1
Great our system has SWAP space now but we need to make is permanent because at this point if we reboot the system the swap space will be lost. What we need to do is add a new entry in out /etc/fstab file. 
echo "/SWAOfile   none    swap    sw    0   0"  /etc/fstab
  • this will make sure that when reboot your system the swap file will be "mounted".
And we are done! There are more to add about Linux SWAP space but we will touch base about this on the next Tutorial - Manage SWAP space Settings in Linux

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