When a Linux server is out of memory, some programs such as MariaDB/MySQL will shutdown automatically. To prevent out of memory (OOM) problem, we can create a swap partition or swap file so as to expand the memory. In this tutorial, we’re going to look at how to create a swap file on Linux server with small memory.
First, let me explain some background information.
When you are installing Linux on a desktop computer or a server, one of the considerations is how much swap space it’s going to use. Swap space is a kind of virtual memory. Linux divides RAM into pages. When the physical RAM starts to fill up, Linux can swap out some pages in the RAM to the swap space on disk. To calculate how much virtual memory your system has, simple add physical RAM and swap space.
Swap space in Linux can be a swap partition, swap file or a combination of them. In windows, it’s just a page file stored in C drive. Normally Linux installers such as Ubuntu ubiquitous and CentOS anaconda try to set aside a swap partition when installing the system.
To check your Linux system’s swap space, use the
swapon --show command. You may need to use
We can get the following information.
- how many swap partitions or swap files are in our Linux system
- the size of each swap device
- how much swap space is being used
- the priority of each swap device
Priority controls which swap device is used first. Swap devices with a higher number are used before swap devices with lower number.
Create a Swap File
On a cloud Linux server, you may have only one partition for the root file system. In this case, you have no way of creating another partition and format it as swap partition. In stead we can create a swap file in the root file system.
First we use
fallocate command to create a file. For example, create a file named swapfile with 512M capacity in root file system:
sudo fallocate -l 512M /swapfile
To create a 1G file:
sudo fallocate -l 1G /swapfile
Then make sure only root can read and write to it.
sudo chmod 600 /swapfile
Format it to swap:
sudo mkswap /swapfile
Setting up swapspace version 1, size = 524284 KiB no label, UUID=h32b3e10-0779-4865-9ea0-6e2af8f3kea9
Enable the swap file
sudo swapon /swapfile
Now you can see that it’s enabled with
swapon --show command.
[email protected]:~$ sudo swapon --show NAME TYPE SIZE USED PRIO /swapfile file 512M 132K -1
To let Linux automatically mount this swap file when booting up, add this line to
/swapfile none swap defaults 0 0
Please note that you need to delimit each column with Tab key.
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