如何在Linux上使用文件作为内存交换区(Swap Area)

交换区域(Swap Area)有什么作用？
交换分区是操作系统在内存不足（或内存较低）时的一种补充。通俗的说，如果说内存是汽油，内存条就相当于油箱，交换区域则相当于备用油箱。

Ubuntu Linux安装时可以在磁盘上划出一个分区用作内存交换区域。文件则介绍如何使用文件作为内存交换区域。这两种方法在使用效果上没有太大区别，但文件可以在分区之后创建，且调整大小更容易，所以这种方案更加灵活。

本文在Ubuntu上试验成功，在其他发行版操作类似。

原文地址：https://www.howtoforge.com/ubuntu-swap-file

How to Add a Swap File On Ubuntu Linux

Contents

1.1 What does Swapping Mean in Linux
1.2 Important: Preliminary Check before setting up a Swap file
1.3 Adding a Swap File on Ubuntu - A Step-By Step Guide
1.3.1 Step 1-Checking the File System
1.3.2: Step 2-Creating the Swap File
1.3.3: Step 3- Enabling the Swap File
1.3.4: Step 4-Adding the Swap to the fstab file
1.3.5: Step 5-Setting Up Appropriate Permission Levels on the Swap File
1.4 Test

1.1-What Does Swapping Mean in Linux?

A swap file enables an operating system (OS) to utilize hard disk space to simulate additional memory. Effectively, as the system goes on the low-memory mode, it swaps a certain section of the RAM that an idle program may be using onto the hard disk to release memory capacity for other vital programs. Subsequently, when the user returns to the swapped out program, it swaps places with another program in RAM. This intelligent mix of RAM and swap files is commonly referred to as virtual memory, and its use allows the system to run a far greater number of programs than possible using only the built-in RAM.

Expressly in the context of Linux, the RAM on the Linux server comprises memory chunks technically termed as pages. To free up these RAM pages, a LINUX swap may be executed to copy the memory page from the parent RAM to a preconfigured space on the hard disk. Linux swaps of this nature allow a system to exploit more physical memory than was initially made available. Though a useful tool, swapping does suffer from a few demerits. Firstly, the activity triggers a sizeable amount of hard disk reading and writing, which invariably slows down your system to a considerable extent. Secondly, swap-thrashing may happen if the system gets inundated with a heavy load of files being swapped in and out frequently.

1.2-Important: Preliminary Check before setting up a Swap file

Before the users proceed with the process of setting up a swap file, it is imperative to check for any existing swap files that may have been enabled on the VPS. This can be done by reviewing the summary of swap usage. The following command shall lead you to the required summary:

sudo swapon -s

The above command shall lead to the following results:

Filename Type Size Used Priority

If the above happens to be an empty list, you may be rest assured that no swap files have been enabled.

1.3-Adding a Swap File on Ubuntu - A Step-By Step Guide

Once the preliminary check has been done, the users are now all geared up for adding a Swap File on Ubuntu. All they need to do is follow the steps given below:

1.3.1: Step 1 - Checking the File System

Once it has been firmly established that there do not exist any active swap files on the virtual server, one must check the amount of space available on the server using the df command. The swap file would normally occupy 256MB of space, which would amount to merely 8% of the /dev/sda on a typical 32GB capacity server, one may assuredly proceed with the process The following set of commands shall help users complete the first step.

df

Filesystem 1K-blocks Used Available Use% Mounted on

/dev/sda 20907056 1437188 18421292 8% /
udev 121588 4 121584 1% /dev
tmpfs 49752 208 49544 1% /run
none 5120 0 5120 0% /run/lock
none 124372 0 124372 0% /run/shm

The above step shows the free space on your harddisk. Ensure that there is enough free space for your swap file. next we will create the actual file with dd as explained below.

1.3.2: Step 2 - Creating the Swap File

The second step comprises creating the Swap File with a size of 256 MB by using the dd command follows:

sudo dd if=/dev/zero of=/swapfile bs=1024 count=256k

of=/swapfile” designates the file’s name. In this case the name is swapfile.

Consequently the swap file can be prepared by creating a designated Linux swap area using the following command:

sudo mkswap /swapfile

The above command shall display the following results:

Setting up swapspace version 1, size = 262140 KiB

no label, UUID=103c4545-5fc5-47f3-a8b3-dfbdb64fd7eb

Completion of Step 2 shall lead to the creation of the Swap File. The next step, as described below, shall lead to the activation of the Swap File thus created.

1.3.3: Step 3 - Enabling the Swap File

The process of creating the Swap File on Ubuntu needs to befinished by enabling the file so created. This can be done by using the following command:

sudo swapon /swapfile

Completion of Step 3 shall enable users to view the new swap file when under swap summary, as follows:

swapon -s

Filename Type Size Used Priority

/swapfile file 262140 0 -1

The file displayed as per details above shall exist on the virtual private server till the time the machine reboots. However, users enjoy the option of making the Swap File permanent by following the next step. 1.3.4: Step 4-Adding the Swap to the fstab file In order to lend permanence to the swap file you created, you must add the same to the fstab file. The following command that will help you open up the file remains the first in the process that shall help you do that:

sudo nano /etc/fstab

Subsequently, users need to paste in the following line:

 /swapfile       none    swap    sw      0       0

Post the above step, the Swappiness in the file must ideally be set to 10, by using the following set of commands:

echo 10 | sudo tee /proc/sys/vm/swappiness

echo vm.swappiness = 10 | sudo tee -a /etc/sysctl.conf

Important: Doing away with the above important sub-step may invariably lead to poor system performance, while configuring the setting to 10 shall lead to the swap acting as an emergency buffer, thus averting irksome out-of-memory crashes.

1.3.5: Step 5 - Setting Up Appropriate Permission Levels on the Swap File

The last and final step in the process of creating a Swap File on Ubuntu remains setting up apt permission levels for the file. This is essential to prevent the file from becoming world-readable and lending appropriate security to the same. The same may be executed by using the following either of the following commands (depending upon the case)::

sudo chown root:root /swapfile

sudo chmod 0600 /swapfile

1.4 Test

Finally we test that the swap file is active. Run the top command

top

the swap space should be shown in the column

KiB Swap: 262140