SWAP memory is used to help RAM, once it cannot store any more data.
The data which cannot be stored in RAM is then stored to SWAP memory in the
hard disk. In this article, we will provide the basic steps on how to modify and
increase SWAP memory in our KVM virtualization Linux VPS packages.
In general, there are different opinions on how much SWAP memory your
server should have. It could be half of your RAM, the same amount or even more
than RAM. In our article, we will assign 6 GB of SWAP to the server.
Allocating SWAP Memory
First thing you should do is to check if there is no SWAP memory in
use on your server with the following command:
Your results will be printed in two lines: "Mem",
"Swap", which will indicate, what the exact amount of RAM and SWAP
memory is on the KVM server. The "Swap" line should only contain
With the following command, we will allocate 6 GB of disk space for our
fallocate -l 6G /swapfile
You can check if your SWAP memory was assigned with this command:
ls -lh /swapfile
At first, your SWAP may not be allocated due to permission issue and
you might be seeing this message:
-rw-r--r-- 1 root root 6.0G Dec 5 14:32 /swapfile
This would suggest we have to make additional changes, first of which
should be changing the permission of the swapfile:
chmod 600 /swapfile
After the change you can check the file permissions again:
ls -lh /swapfile
The results should change as well, comparing to the previous above:
-rw------- 1 root root 6.0G Dec 5 14:36 /swapfile
Now set the swapfile as Linux swap area:
Finally, enable SWAP usage:
We can now check if the SWAP memory was allocated correctly:
Your results will print two lines again, just this time, you will see
a line "Swap" having a variable of 6 GB.
In general, your SWAP memory allocation may stop working after you
reboot the KVM server, so in order to save these
changes permanently, we have to edit /etc/fstab. But first, we would recommend
making a backup of this file:
cp /etc/fstab /etc/fstab.old
Once the backup is done, we have to make the changes to the actual file:
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab
There are few extra options for SWAP memory, which we would like to
address as well:
Option - swappiness, is used to describe when the system will move data
to the SWAP file. The option may have a value from 0 to 100. Closer to 0, means
that your data will be moved to SWAP only when it will be necessary. Closer to 100 means that, data will be moved
to SWAP more often, therefore leaving RAM memory more free. We would recommend
keeping this option closer to 0, for example, 10.
You can check the current value with the following command:
Option - vfs_cache_pressure, this option sets how often the information
about the file system is updated. By default, it should be 100, but we would recommend
using a lower value,
for example, 50.
The current value can be checked with this command:
Both of the additional options can be edited at the file
by adding the following line at the bottom of the file:
If any questions remain, please
contact our customer support, we will be glad to help.