How to create and activate Swap

Swap is virtual memory concept used on Linux operating system. When VPS/Server is in operation, if it runs out of RAM, the system will automatically use part of the hard drive as a memory for applications to operate.

With servers without swap, when running out of RAM, the system automatically stops the MySQL service, so the error message Establishing a Database Connection often appears.

Because the use of a hard drive is slower than RAM, especially for servers that use hard drives that often don’t use SSDs, so not regularly using Swap will reduce system performance. If the situation of running out of RAM often happens, you should optimize the server, increase the cache and upgrade more RAM.

With VPS using OpenVZ virtualization technology, you may not be able to create swap because the system has automatically activated it.

Instructions for creating Swap

Applicable both on CentOS and Ubuntu.

1. Check swap

Before proceeding to create a swap file, you need to check if the system has now enabled swap or not by running:

swapon -s

If there is no information or returns as shown below, the server does not have a swap and you can create.

2. Check free space

Use command  df -h to check free space. 13GB left, still comfortable to create swap:

3. Create swap

– Run the dd command. Here I create 1GB swap (count=1024k) for VPS 768MB RAM

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

The above command will create a 1Gb swap. You can replace count=1024k with  count=2048k … to create a 2Gb swap. The maximum Swap capacity should only be twice the physical RAM.

– Create swap partition

mkswap /swapfile

– Enable swap

swapon /swapfile

This section may encounter a warning  insecure permissions 0644 message , it’s okay, you can just ignore it. We will correct it in the next step.

– Check the swap status, you will see the following:

swapon -s

– Set automatic swap to be activated every reboot

echo /swapfile none swap defaults 0 0 >> /etc/fstab

– Secure swap file by chmod

chown root:root /swapfile
chmod 0600 /swapfile

4. Configure Swappiness

Swappiness is the priority of using swap, when the amount of RAM remaining equals the value of swappiness (in percentage), the swap will be used. Swappiness has a value between 0 – 100.

• swappiness = 0: swap is only used when RAM is used up.
• swappiness = 10: swap is used when the RAM is 10%.
• swappiness = 60: swap is used when the RAM is 60%.
• swappiness = 100: swap takes precedence as RAM.

Since data processing speed on RAM is much higher than Swap, you should set this value close to 0 to get the most out of system power. It is best to set it to 10.

– Check the system’s swap file usage by running the following command line

cat /proc/sys/vm/swappiness

– Adjust the swappiness parameter using the command sysctl

sysctl vm.swappiness=10

– Check again you will see the result returned 10

cat /proc/sys/vm/swappiness

To ensure that this parameter remains the same every time you restart your VPS, you need to adjust the parameter vm.swappiness at the end of the file /etc/sysctl.conf(if you don’t have it, add it manually).

nano /etc/sysctl.conf

Add the following line at the end if it is not there, if it is, update the value:

vm.swappiness = 10

Press Ctrl + O to save, Enter and Ctrl + X to exit.

– Restart VPS and check the results again:

swapon -s
cat /proc/sys/vm/swappiness

Note

– With CentOS 7.2 can tune profile will overwrite  vm.swappinessevery reboot, you need to check which profile is overwrite to reconfigure manually.

grep vm.swappiness /usr/lib/tuned/*/tuned.conf

Output can be as follows:

[root@server ~]# grep vm.swappiness /usr/lib/tuned/*/tuned.conf
/usr/lib/tuned/latency-performance/tuned.conf:vm.swappiness=10
/usr/lib/tuned/throughput-performance/tuned.conf:vm.swappiness=10
/usr/lib/tuned/virtual-guest/tuned.conf:vm.swappiness = 30

At this point, open the file  /usr/lib/tuned/virtual-guest/tuned.conf, find  vm.swappinessand change it vm.swappiness = 30 to  vm.swappiness = 10.

nano /usr/lib/tuned/virtual-guest/tuned.conf

Press Ctrl + O to save, Enter and Ctrl + X to exit.

Restart the VPS and check the results again.

Change swap size

If you followed the instructions above to create a swap file and want to change the swap size, reverse the process.

– Disable Swap

swapoff /swapfile

– Delete file Swap

rm -f /swapfile

– Create a new swap file with the desired size. For example, I created 2GB (2048k)

sudo dd if=/dev/zero of=/swapfile bs=1024 count=2048k

– Create Swap partition

mkswap /swapfile

– Enable Swap

swapon /swapfile

– Secure swap file by chmod

chown root:root /swapfile
chmod 0600 /swapfile

– Check swap status

swapon -s

When changing the swap size, the previous settings such as automatically activated when the server reboots, the Swappiness parameters are preserved, so you do not need to do anything else.

Now restart the server and check the results again.

Thanks to hocvps to initiate this article!