How To Add Swap Space on CentOS 7

Linux 的 Swap 類似 Windows 下的虛擬記憶體, 當記憶體不足時, 把 Swap 的空間當作暫時記憶體使用, 從而解決記憶體不足的問題。如果 Swap 空間不夠用, 需要增加 Swap 空間的話, 可以使用 Swap Partition 或 Swap 檔案實現, 以下會介紹在 RHEL, CentOS 及 Fedora 透過 Swap 檔案增加 Swap 空間。

在開始前先檢查一些目前系統是否有設定 Swap 空間, 可以用 "swapon -s" 指令檢查:

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# swapon -s Filename        Type    Size  Used  Priority /dev/sda2                               partition 2097148 38516 -1

上看到目前系統有 2 GB Swap 空間, 使用了 37MB.

然後再看看系統各分割區的可用空間:

# df -h

假設我要在 / 下面建立 4GB 的 Swap 檔案, 可以用 fallocate 指令, 語法如下:

# fallocate -l 4G /swapfile

建立好 /swapfile 檔案後, 然後需要格式化這個 Swap 檔案, 並設定系統使用這個檔案作為 Swap 空間:

# chmod 600 /swapfile
# mkswap /swapfile
# swapon /swapfile

設定完成後, 可以再用 "swapon -s" 檢查設定是否生效:

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# swapon -s Filename                Type        Size    Used    Priority /swapfile               file        4194300 0     -1

現在系統已經可以使用 /swapfile 用為 Swap 空間, 但當重新啟動系統後, 設定就會失效, 要在重新開機後自動使用 /swapfile 作為 Swap 空間, 需要修改 /etc/fstab 檔案:

# vi /etc/fstab

在最低下加下這行:

1	/swapfile   swap    swap    sw  0   0

儲存後就完成設定了。

Swap is a space on a disk that is used when the amount of physical RAM memory is full. When a Linux system runs out of RAM, inactive pages are moved from the RAM to the swap space.

Swap space can take the form of either a dedicated swap partition or a swap file. In most cases when running CentOS on a virtual machine a swap partition is not present so the only option is to create a swap file.

This tutorial explains how to add a swap file on CentOS 7 systems.

Before You Begin

Before proceeding with this tutorial, check if your CentOS installation already has swap enabled by typing:

sudo swapon --show

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If the output is empty, it means that your system does not have swap space enabled.

Otherwise if you get something like below, you already have swap enabled on your machine.

NAME      TYPE      SIZE USED PRIO
/dev/dm-1 partition 1.5G   0B   -1

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Although possible, it is not common to have multiple swap spaces on a single machine.

Creating a Swap File

The user you are logged in as must have sudo privileges to be able to activate swap. In this guide, we will add 1G of swap, if you want to add more swap, replace 1G with the size of the swap space you need.

Follow the steps below to add swap space on a CentOS 7 system.

1. First, create a file which will be used as swap space:

   sudo fallocate -l 1G /swapfile

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   If the fallocate utility is not available on your system or you get an error message saying fallocate failed: Operation not supported, use the following command to create the swap file:

   sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576

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2. Ensure that only the root user can read and write the swap file:

   sudo chmod 600 /swapfile

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3. Next, set up a Linux swap area on the file:

   sudo mkswap /swapfile

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4. Run the following command to activate the swap:

   sudo swapon /swapfile

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   Make the change permanent by opening the /etc/fstab file:

   sudo nano /etc/fstab

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   and pasting the following line:
   /etc/fstab

   /swapfile swap swap defaults 0 0

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5. Verify that the swap is active by using either the swapon or the free command as shown below:

   sudo swapon --show

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   NAME      TYPE  SIZE   USED PRIO
   /swapfile file 1024M 507.4M   -1

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   sudo free -h

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                 total        used        free      shared  buff/cache   available
   Mem:           488M        158M         83M        2.3M        246M        217M
   Swap:          1.0G        506M        517M

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Adjusting the Swappiness Value

Swappiness is a Linux kernel property that defines how often the system will use the swap space. Swappiness can have a value between 0 and 100. A low value will make the kernel to try to avoid swapping whenever possible while a higher value will make the kernel to use the swap space more aggressively.

The default swappiness value on CentOS 7 is 30. You can check the current swappiness value by typing the following command:

cat /proc/sys/vm/swappiness

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30

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While the swappiness value of 30 is OK for desktop and development machines, for production servers you may need to set a lower value.

For example, to set the swappiness value to 10, type:

sudo sysctl vm.swappiness=10

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To make this parameter persistent across reboots append the following line to the /etc/sysctl.conf file:

/etc/sysctl.conf

vm.swappiness=10

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The optimal swappiness value depends on your system workload and how the memory is being used. You should adjust this parameter in small increments to find an optimal value.

Removing a Swap File

To deactivate and remove the swap file, follow these steps:

1. Start by deactivating the swap space by typing:

   sudo swapoff -v /swapfile

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2. Next, remove the swap file entry /swapfile swap swap defaults 0 0 from the /etc/fstabfile.
3. Finally, delete the actual swapfile file:

   sudo rm /swapfile

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Conclusion

You have learned how to create a swap file and activate and configure swap space on your CentOS 7 system.

If you hit a problem or have feedback, leave a comment below.