How to add swap space on CentOS

What is swap space/memory?

When the amount of physical memory (RAM) is full, swap space in Linux is used. Non-active pages in memory are moved to the swap memory if the system requires more memory resources and also the RAM is complete.

Swap space can take the form of either a dedicated swap partition or a swap file. Typically, when running CentOS on a virtual machine, a swap partition is not present, and the only option is to create a swap file.

Before You Begin

Swap should not be seen as a replacement to physical memory. Since swap space is a section of the hard drive, it has a slower access time than physical memory. If your system constantly runs out of memory, you should add more RAM.

• How to calculate the needed swap space in CentOS?

Generally, the size of the swap file depends on how much RAM your system has:

The swap space in CentOS should equal 2x physical RAM for up to 2 GB of physical RAM, and then an additional 1x physical RAM for any amount above 2 GB, but never less than 32 MB. For CentOS systems with really large amounts of RAM (more than 32 GB), you can likely get away with a smaller swap partition (around 1/2 half the physical RAM).

// M = Amount of RAM in GB.
// S = Amount of Swap Memory in GB.

if (M < 2):
   S = M * 2
else if (M > 32):
  S = M / 2
else:
  S = M

So we can say:

1. A system with 2 GB of physical RAM would have 4 GB of swap memory.
2. A system with 3 GB of physical RAM would have 3 GB of swap memory.
3. A system with 32 GB of physical RAM would have 16 GB of swap memory.

Creating a large swap space partition can be especially helpful if you plan to upgrade your RAM at a later time.

• How to check if my CentOS installation already has swap enabled ?

Before you start, check if your CentOS installation already has swap enabled by typing: cat /proc/swaps

[root@krython ~]# cat /proc/swaps
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2843600 0       -2

Alternatively, use the swapon --show command:

[root@krython ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2843600 0       -2

If the outcome is empty it means that your system does not have swap space enabled.

Creating the swap file

The user that you are logged in need to be a root user or need to have sudo privileges to be able to include and also create swap space.

We will add 2GB of Swap memory to our CentOS System.

Let's begin:

1. Create the swap file that will be used as a Swap space:

sudo dd if=/dev/zero of=/swapfile bs=1024 count=2097152

2. Ensure that just the root user can read and also write the swap file by setting the correct permissions:

sudo chmod 600 /swapfile

3. Set up the file as Linux swap area:

sudo mkswap /swapfile

4. Activate the swap space A short-lived turn on (lost on reboot):

sudo swapon /swapfile

5. Customize the /etc/fstab to immediately add the new swap at system boot time by adding the following line:

/swapfile swap swap defaults 0 0

6. Confirm that the swap is active by using either the swapon or the free command:

[root@krython ~]# sudo swapon -s
NAME                 TYPE      SIZE USED PRIO
/swapfile partition 2.1G   0B   -2

[root@krython ~]# free -m
              total        used        free      shared  buff/cache   available
Mem:            809         160         258           5         389         521
Swap:          2195           0        2195

What is swappiness, how to change it's value?

Swappiness is a Linux kernel property that sets the balance between swapping out pages from the physical memory to the swap space and removing pages from the page cache. It basically defines how often the system will use the swap space.

• swappiness can have a value of between 0 as well as 100
• swappiness=0 tells the kernel to avoid swapping processes out of physical memory for as long as feasible
• swappiness=100 informs the kernel to boldy swap processes out of physical memory as well as relocate them to swap cache

The default setup in CentOS is swappiness=30.

Lowering the default value of swappiness will possibly boost general performance for a regular CentOS desktop setup.

A value of swappiness=10 is suggested, yet feel free to experiment.

Remember: CentOS server installments have different performance requirements to desktop systems, and likewise the default value of 30 is likely preferable.

1. Check the swappiness value:

cat /proc/sys/vm/swappiness

2. Change the swappiness value A temporary change (lost on reboot):

sudo sysctl vm.swappiness=60

3. Make the change permanent, edit the configuration file located in /etc/sysctl.conf .
4. Search for vm.swappiness and change its value as desired. If vm.swappiness does not exist, add it to the end of the file like so:

vm.swappiness=60

5. Finally, reboot your system:

sudo reboot

Remove a swap file

You will do the opposite steps of the setup process, deactivate swap space then remove the swap file

1. Deactivating the swap space:

sudo swapoff -v /swapfile

2. Remove the swap file entry from the /etc/fstab file:

/swapfile swap swap defaults 0 0

3. Finally, Delete the swap file:

sudo rm -rf /swapfile

Conclusion

1. You have learned how to create a swap file and also activate it on your CentOS system.
2. You have learned how to change the swappiness value on your CentOS system.
3. You have learned how to deactivate swap space as well as remove your swap file on your CentOS system.

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