What is a swap file on Ubuntu server or desktop system?
As a sysadmin it is necessary to add more swap space after installation on the server. Swap file allows Ubuntu Linux to use hard disk to increase virtual memory.
Virtual Memory = RAM + Swap space/file
Virtual Memory (1GB) = Actual RAM (512MB) + Swap space/file (512MB)
When the Ubuntu server runs low on memory, it swaps a section of RAM (say an idle program like foo) onto the hard disk (swap space) to free up memory for other programs. Then when you need that program (say foo again), kernel swapped out foo program, it changes places with another program in RAM.
Procedure to add a swap file on a Ubuntu Linux
Open the Terminal app or use the ssh client to get into the remote server. Login as a root user using sudo command:
sudo -s
Create a swap file command
Type the following command to create a 2GB swap file on Ubuntu:# dd if=/dev/zero of=/swapfile bs=1G count=2
Sample outputs:
2+0 records in
2+0 records out
2147483648 bytes (2.1 GB) copied, 20.2256 s, 106 MB/s
Verify that file has been created on the server:# ls -lh /swapfile
Sample outputs:
-rw-r--r-- 1 root root 2.0G Oct 29 14:07 /swapfile
Creating swap space using fallocate command instead of dd command
-rw-r--r-- 1 root root 1.0G Oct 29 14:11 /swapfile-1
Secure the swap file
Type the following chmod command and chown command to secure and set correct file permission for security reasons:# chown root:root /swapfile
# chmod 0600 /swapfile
# ls -lh /swapfile
Sample outputs:
-rw------- 1 root root 2.0G Oct 29 14:07 /swapfile
A world-readable swap file is a huge local vulnerability. The above commands make sure only root user can read and write to the file.
Turn on the swap file
First, use the mkswap command as follows to enable the swap space on Ubuntu:# mkswap /swapfile
Sample outputs:
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=10231c61-6e55-4dd3-8324-9e2a892e7137
Finally, activate the swap file, enter:# swapon /swapfile
Verify new swap file and settings on Ubuntu
Type the following command# swapon -s
Sample outputs:
Filename Type Size Used Priority
/dev/sda5 partition 3998716 704 -1
/swapfile file 2097148 0 -2
You can also run the following commands to verify swap file and its usage:# grep -i --color swap /proc/meminfo
# top
# htop
# atop
How can I disable swapfile on Ubuntu?
You need to use the swapoff command as follows:# swapoff /swapfile
# swapon -s
Update /etc/fstab file
You need to make sure the swap file enabled when server comes on line after the reboot. Edit /etc/fstab file, enter:# vi /etc/fstab
Append the following line:
/swapfile none swap sw 0 0
Save and close the file.
Tuning the swap file i.e. tuning virtual memory
You can tune the following two settings:
- swappiness
- min_free_kbytes
- vfs_cache_pressure
How do I set swappiness on a Ubuntu server?
The syntax is:# sysctl vm.swappiness=VALUE
# sysctl vm.swappiness=20
OR# echo VALUE > /proc/sys/vm/swappiness
# echo 30 > /proc/sys/vm/swappiness
The value in /proc/sys/vm/swappiness file controls how aggressively the kernel will swap memory pages. Higher values increase agressiveness, lower values descrease aggressiveness. The default value is 60. To make changes permanent add the following line to/etc/sysctl.conf:
echo 'vm.swappiness=30' >> /etc/sysctl.conf
For database server such as Oracle or MySQL I suggest you set a swappiness value of 10.
