Introduction

What Is Swap

Swap in a Linux system, also known as swap space, is similar to the virtual memory (pagefile.sys) of Windows. When memory runs low, a portion of the disk space is turned into virtual memory to store data that is not being used at the moment.

Setup

  1. Check the current state

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    free -m
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                 total       used       free     shared    buffers     cached
    Mem:          3952       2035       1916          9        217       1392
    -/+ buffers/cache:        425       3526
    Swap:            0          0          0

    As you can see, Swap is not enabled. Below we will increase it to match the memory size (4G).

  2. Create the Swap file

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    mkdir swap

    cd swap

    sudo dd if=/dev/zero of=swapfile bs=1024 count=4M   # bs is the block size, count is the number of blocks; 1024 * 4M = 4G
    # 4194304+0 records in
    # 4194304+0 records out
    # 4294967296 bytes (4.3 GB) copied, 88.3999 s, 48.6 MB/s

    Convert the file into a swap file.

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    sudo mkswap -f  swapfile 
    # Setting up swapspace version 1, size = 4194300 KiB
    # no label, UUID=bebbcbad-dda2-49f9-9aab-4b24b1d62d87

  3. Activate Swap

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    sudo swapon swapfile

    Verify the activation.

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    free -m  
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                 total       used       free     shared    buffers     cached
    Mem:          3952       3842        109          9          1       3369
    -/+ buffers/cache:        470       3481
    Swap:         4095          0       4095

  4. Configuration

    • Adjust swappiness
      swappiness is a value from 0 to 100. A higher value means the system will more actively use Swap.

      • Temporary change
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        sudo sysctl vm.swappiness=40
      • Permanent change 
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        sudo vim /etc/sysctl.conf
        # Add a line
        vm.swappiness = 40

    • Change permissions
      Set the file so that only the root user has read and write permissions.

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      sudo chown root:root /swap/swapfile
      sudo chmod 0600 /swap/swapfile

  5. Enable on boot

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    sudo vim /etc/fstab

    Add the following line at the end of the file.

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    /swap/swapfile       none    swap    sw      0       0