SWAP in Linux

Guide to creating a SWAP partition on Linux

SWAP is a virtual memory mechanism where part of the data from RAM is offloaded to a storage device such as an HDD (hard disk drive) or SSD (solid-state drive).

In Linux, RAM is divided into blocks called pages. Swapping is the process of copying these memory pages into a specially allocated area on disk called swap space (this can be either a dedicated disk partition or a swap file). The combined size of physical RAM and swap space defines the total amount of available virtual memory.

Why swapping is needed

When the system requires more memory than is currently available, the kernel moves the least-used pages into swap, freeing RAM for active processes.
Many memory pages used by applications during startup are no longer needed afterward. The system can push these to swap, freeing up RAM.

Drawbacks of SWAP

Accessing disk storage is much slower than RAM: operations in RAM are measured in nanoseconds, while on a disk they are measured in milliseconds. That means the same operation can take tens of thousands of times longer on disk. As a result, the more pages are swapped, the slower the system performs.

Note

When applications start relying heavily on swap, performance can degrade significantly. At that point, it’s often better to increase available memory. One option is migrating your project to a VPS or a dedicated server, where you can scale resources more flexibly.

Databases are usually the first to be affected by memory shortages. Typical symptoms include:

The website crashing with a “Database connection error” (for example, when MySQL stops responding).
The log file /var/log/mysql.log showing InnoDB: Fatal error: cannot allocate memory for the buffer pool, which indicates that there isn’t enough RAM available to allocate the buffer.

If swap is located on an SSD, keep in mind that data still has to go through the storage subsystem, which increases latency.

Checking if swap is enabled

swapon -s
Filename				Type		Size	Used	Priority
/swapfile				file		40956	40956	-1

You can also run:

free -m
              total        used        free      shared  buff/cache   available
Mem:            488         160           7          40         320         259
Swap:            39          39           0

Here we can see that swap is active and will be used when needed.

Creating a swap file (if missing)

Create a 512 MB swap file:

dd if=/dev/zero of=/swap.file bs=1M count=512

To change the size, adjust the count value.

Set the correct permissions so only root can read and write:

chmod 600 /swap.file

Format the file as swap:

mkswap /swap.file

Edit /etc/fstab to make the swap file permanent:

/swap.file      swap            swap    defaults        0       0

After rebooting, verify that swap is active with:

free -m