SWAP
Swap is an additional storage space in Linux systems that is used as virtual memory when the physical RAM capacity is exhausted. Swap can be a dedicated partition or or swap file, and serves as a memory reserve that allows the system to keep running even if the memory load increases.
Although swap is much slower than RAM, it is very important in the following situations:
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Handling high memory loads when RAM is full.
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Provides spare space for background or idle processes.
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Prevents applications from crashing due to memory exhaustion.
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Supports the hibernation feature (if used).
On production servers, properly configured swap can improve system stability, but it must be used wisely because swap over disk (especially HDD) has a much slower read/write speed than RAM.
Swap Check
Before doing swap configuration such as adding, deleting, or creating swaps, you should check first.
Method 1:
swapon --show
If swap is on, you will see output like this. If it is not active, there will be no output:
Filename Type Size Used Priority
/dev/dm-1 partition 4194300 0 -2
Method 2:
free -h
Output example:
total used free shared buff/cache available
Mem: 1.7Gi 178Mi 1.1Gi 8.0Mi 448Mi 1.4Gi
Swap: 0B 0B 0B
The Swap: line indicates the size and usage of the swap. If the value is 0 then there is no swap.
Create SWAP File
Create a swap equal to 2x the total RAM, for example the current RAM is 2GB then the swap is made 4GB
fallocate -l 4G /swap
Adjust permission 0600 for security reasons
chmod 0600 /swap
Create swap
mkswap /swap
Activate swap
swapon /swap
Verify
swapon --show
Output example:
NAME TYPE SIZE USED PRIO
/swap file 4G 0B -2
Enable swap permanently on reboot
echo '/swap none swap sw 0 0' >> /etc/fstab
Verify:
cat /etc/fstab
Increase Swap Size
It is assumed that the swap was created earlier with the swapfile /swap.
Shut down the current swap and delete the swap file:
swapoff /swap
rm -rf /swap
Recreate the swap with a new size such as 6GB:
fallocate -l 6G /swap
chmod 0600 /swap
mkswap /swap
swapon /swap
Enable swap permanently on reboot
echo '/swap none swap sw 0 0' >> /etc/fstab
Disable and Remove Swap Completely
swapoff /swap
sed -i '/\/swapfile/d' /etc/fstab
rm -f /swap
Verify:
swapon --show
cat /etc/fstab
Swappiness
Swappiness is a parameter in the Linux kernel that determines how aggressively the system uses swap when RAM starts to fill up. This value influences the system's decision whether to keep data in RAM or move it to swap (virtual memory on disk).
| Value | Means |
|---|---|
0 | Use swap only if RAM is completely exhausted. |
10 | Use as much RAM as possible, swap only when urgent. |
60 | Default in many distros - balanced between RAM and swap. |
100 | Use swap as soon as possible, as if RAM and swap were equivalent. |
How to check and configure swappiness
Check the current value:
cat /proc/sys/vm/swappiness
Change temporarily until the next reboot:
sysctl vm.swappiness=10
Permanently change and apply the configuration. syctl -p applies the settings immediately without rebooting:
echo 'vm.swappiness=10' >> /etc/sysctl.conf
sysctl -p
Verify after reloading the sysctl configuration and the output should show the value 10.
Flush Cache RAM
Linux automatically caches frequently used files, disks, and inodes into RAM. This cache is “use when necessary” and is not immediately emptied even if the application no longer needs the data. By dropping the cache, you force the system to dump the cache to free up RAM.
It is not recommended to run this regularly on production servers, as the cache helps improve performance. Clearing the cache too often can actually decrease performance ( all processes must re-access the disk).
Run the following command:
sync; echo 1 > /proc/sys/vm/drop_caches && /sbin/swapoff -a && /sbin/swapon -a
Create an automation script to run this ram flush via a cron job:
nano /root/flush-cache.sh
Script content:
#!/bin/bash
sync; echo 1 > /proc/sys/vm/drop_caches && /sbin/swapoff -a && /sbin/swapon -a
echo "[ $(date) ] Cache & swap flushed" >> /var/log/flush-cache.log
Set permission:
chmod +x /root/flush-cache.sh
Then add it to Cron:
crontab -e
Add the following parameters to run the script every day at 02:00 AM:
0 2 * * * /root/flush-cache.sh
Check the logs:
cat /var/log/flush-memory.log
Output example:
[ Thu Jul 3 09:02:18 PM WIB 2025 ] Cache & swap flushed