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How to create extra swap file – Linux Command Line?

What is SWAP?

It’s simply a portion of hard disk, which acts like RAM. It’s used when total amount of Physical Memory is full. If the RAM is filled up and the system requires more memory, inactive pages on the memory (RAM) are moved to the SWAP space. Even it acts like physical memory, it’s not a replacement for RAM.

Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files.

A better equation for SWAP:

SWAP = Physical Memory x 2 ; if the Physical Memory < 2 
SWAP = Physical Memory + 2 ; if the Physical Memory > 2

Example:

If RAM = 2
Swap = 2 x 2 = 4

If RAM = 4 (> 2)
Swap = 4 + 2 = 6

Here I’m explaining the steps to create Swap space by creating Swap file using dd command and swapon.

Procedures

** dd command is used to create a swap file.
** mkswap command is used to setup a Linux swap area under the system or file.

Step 1: Login as root user.
Step 2: Create a storage file.(using dd command)

dd if=/dev/zero of=/swapfile1 bs=1024 count=524288

Output should be like:

# dd if=/dev/zero of=/swapfile1 bs=1024 count=524288
524288+0 records in
524288+0 records out
536870912 bytes (537 MB) copied, 1.34966 s, 398 MB/s

Command explanation:

1. if=/dev/zero : Read from /dev/zero file. /dev/zero is a special file in that provides as many null characters to build storage file called /swapfile1.
2. of=/swapfile1 : Read from /dev/zero write stoage file to /swapfile1.
3. bs=1024 : Read and write 1024 BYTES bytes at a time.
4. count=524288 : Copy only 523288 BLOCKS input blocks.

step 3: Setup Linux swap area (using “mkswap” command).

mkswap /swapfile1

setup correct file permission for better security.

chown root.root /swapfile1
chmod 600 /swapfile1

Step 4: Activate the swap file (using swapon command).

swapon /swapfile1
Note : To de-activate swap using the command swapoff

SWAP

That’s it dude!! 🙂

For permanent setting, add this to /etc/fstab file

vi /etc/fstab
####
/swapfile1 swap swap defaults 0 0
####

Save and close the file! It came automatically after the next reboot of your server. Use “free -m” command to check the SWAP details. Cool… 🙂

Arunlal Ashok

Sr. Linux Server Administrator. I'm managing Linux servers since 2012. I started this blog to share and discuss my ideas. Check My Profile in uPwork (oDesk) and create a job, if you need any administration help. Thanks!!

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2 Responses

  1. raymond says:

    Thank you for the shared knowledge, can this solution be applied to

    Out of memory: ⚠ The process “mysqld” was terminated because the system is low on memory.

    mysqld

    Event Time

    Thursday, February 11, 2016 8:12:55 PM UTC

    Process Username

    mysql

    Process Total Virtual Memory

    1371440kB

    Process Anonymous Resident Set Size

    96292kB

    Process File Resident Set Size

    312kB

    Process OOM Score

    38

    Status

    Out of Memory ⚠

    Memory Information

    Used

    443 MB

    Available

    2.96 GB

    Installed

    3.4 GB

    Uptime

    36 days, 3 hours, 43 minutes, and 29 seconds

    IOStat Information

    • Arunlal A says:

      I’m not sure if the SWAP helps here.
      Please make sure that the PHP memory limit and also try removing cached memory. This may help.

      If the problem persists, paste here the output of “free -m.”

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