How To Add Swap Space on Debian 8

Introduction

One of the easiest way of guarding against out-of-memory errors in applications is to add some swap space to your server. In this guide, we will cover how to add a swap file to a Debian 8 server.

What is Swap?

Swap is an area on a hard drive that has been designated as a place where the operating system can temporarily store data that it can no longer hold in RAM. Basically, this gives you the ability to increase the amount of information that your server can keep in its working “memory”, with some caveats. The swap space on the hard drive will be used mainly when there is no longer sufficient space in RAM to hold in-use application data.

The information written to disk will be significantly slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a fallback for when your system’s RAM is depleted can be a good safety net against out-of-memory exceptions on systems with non-SSD storage available.

Step 1 – Checking the System for Swap Information

Before we begin, we can check if the system already has some swap space available. It is possible to have multiple swap files or swap partitions, but generally one should be enough.

We can see if the system has any configured swap by typing:

sudo swapon --show

If you don’t get back any output, this means your system does not have swap space available currently.

You can verify that there is no active swap using the free utility:

free -h

Output
             total       used       free     shared    buffers     cached
Mem:          1.0G       331M       668M       4.3M        11M       276M
-/+ buffers/cache:        44M       955M
Swap:           0B         0B         0B

As you can see in the Swap row of the output, no swap is active on the system.

Step 2 – Checking Available Space on the Hard Drive Partition

Before we create our swap file, we’ll check our current disk usage to make sure we have enough space. Do this by entering:

df -h

Output
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        25G  946M   23G   4% /
udev             10M     0   10M   0% /dev
tmpfs           201M  4.3M  196M   3% /run
tmpfs           501M     0  501M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           501M     0  501M   0% /sys/fs/cgroup
tmpfs           101M     0  101M   0% /run/user/1001

The device with / in the Mounted on column is our disk in this case. We have plenty of space available in this example (only 946M used). Your usage will probably be different.

Although there are many opinions about the appropriate size of a swap space, it really depends on your personal preferences and your application requirements. Generally, an amount equal to or double the amount of RAM on your system is a good starting point. Another good rule of thumb is that anything over 4G of swap is probably unnecessary if you are just using it as a RAM fallback.

Step 3 – Creating a Swap File

Now that we know our available hard drive space, we can go about creating a swap file within our filesystem.

We will create a file called swapfile in our root (/) directory. The file must allocate the amount of space we want for our swap file. There are two main ways of doing this:

The Traditional, Slow Way

Traditionally, we would create a file with preallocated space by using the dd command. This versatile disk utility writes from one location to another location.

We can use this to write zeros to the file from a special device in Linux systems located at /dev/zero that just spits out as many zeros as requested.

We specify the file size by using a combination of bs for block size and count for the number of blocks. What we assign to each parameter is almost entirely arbitrary. What matters is what the product of multiplying them turns out to be.

For instance, in our example, we’re looking to create a 1 Gigabyte file. We can do this by specifying a block size of 1 megabyte and a count of 1024:

sudo dd if=/dev/zero of=/swapfile bs=1M count=1024

Output
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 1.36622 s, 786 MB/s

Check your command before pressing ENTER because this has the potential to destroy data if you point the of (which stands for output file) to the wrong location.

We can see that 1 Gigabyte has been allocated by typing:

ls -lh /swapfile

Output
-rw-r--r-- 1 root root 1.0G May 30 15:07 /swapfile

If you’ve completed the command above, you may notice that it took a few seconds. Only 1.3 seconds for this small swapfile, but that could increase significantly for larger files on slower hard drives.

If you want to learn how to create the file faster, remove the file swapfile using sudo rm /swapfile, then follow along below:

The Faster Way

The quicker way of getting the same file is by using the fallocate program. Note that this command only works with more modern filesystems, so if you’re using an ext3 system, for instance, this option is not available to you.

The fallocate command creates a file of a preallocated size instantly, without actually having to write dummy contents.

We can create a 1 Gigabyte file by typing:

sudo fallocate -l 1G /swapfile

The prompt will be returned to you almost immediately. We can verify that the correct amount of space was reserved by typing:

ls -lh /swapfile

Output
-rw-r--r-- 1 root root 1.0G May 30 15:07 /swapfile

As you can see, our file is created with the correct amount of space set aside.

Step 4 – Enabling the Swap File

Now that we have a file of the correct size available, we need to actually turn this into swap space.

First, we need to lock down the permissions of the file so that only the users with root privileges can read the contents. This prevents normal users from being able to access the file, which would have significant security implications.

Make the file only accessible to root by typing:

sudo chmod 600 /swapfile

Verify the permissions change by typing:

ls -lh /swapfile

Output
-rw------- 1 root root 1.0G May 29 17:34 /swapfile

As you can see, only the root user has the read and write flags enabled.

We can now mark the file as swap space by typing:

sudo mkswap /swapfile

Output
Setting up swapspace version 1, size = 1048572 KiB
no label, UUID=757ee0b7-db04-46bd-aafb-adf6954ea077

After marking the file, we can enable the swap file, allowing our system to start utilizing it:

sudo swapon /swapfile

Verify that the swap is available by typing:

sudo swapon --show

Output
NAME      TYPE  SIZE USED PRIO
/swapfile file 1024M   0B   -1

We can check the output of the free utility again to corroborate our findings:

free -h

Output
             total       used       free     shared    buffers     cached
Mem:          1.0G       925M        74M       4.3M        13M       848M
-/+ buffers/cache:        63M       936M
Swap:         1.0G         0B       1.0G

Our swap has been set up successfully and our operating system will begin to use it as necessary.

Step 5 – Making the Swap File Permanent

Our recent changes have enabled the swap file for the current session. However, if we reboot, the server will not retain the swap settings automatically. We can change this by adding the swap file to our /etc/fstab file.

Back up the /etc/fstab file in case anything goes wrong:

sudo cp /etc/fstab /etc/fstab.bak

Add the swap file information to the end of your /etc/fstab file by typing:

echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Next we’ll review some settings we can update to tune our swap space.

Step 6 – Tuning your Swap Settings

There are a few options that you can configure that will have an impact on your system’s performance when dealing with swap.

Adjusting the Swappiness Property

The swappiness parameter configures how often your system swaps data out of RAM to the swap space. This is a value between 0 and 100 that represents a percentage.

With values close to zero, the kernel will not swap data to the disk unless absolutely necessary. Remember, interactions with the swap file are “expensive” in that they take a lot longer than interactions with RAM and they can cause a significant reduction in performance. Telling the system not to rely on the swap much will generally make your system faster.

Values that are closer to 100 will try to put more data into swap in an effort to keep more RAM space free. Depending on your applications’ memory profile or what you are using your server for, this might be better in some cases.

We can see the current swappiness value by typing:

cat /proc/sys/vm/swappiness

Output
60

For a Desktop, a swappiness setting of 60 is not a bad value. For a server, you might want to move it closer to 0.

We can set the swappiness to a different value by using the sysctl command.

For instance, to set the swappiness to 10, we could type:

sudo sysctl vm.swappiness=10

Output
vm.swappiness = 10

This setting will persist until the next reboot. We can set this value automatically at restart by adding the line to our /etc/sysctl.conf file:

sudo nano /etc/sysctl.conf

At the bottom, you can add:

vm.swappiness=10

Save and close the file when you are finished.

Adjusting the Cache Pressure Setting

Another related value that you might want to modify is the vfs_cache_pressure. This setting configures how much the system will choose to cache inode and dentry information over other data.

Basically, this is access data about the filesystem. This is generally very costly to look up and very frequently requested, so it’s an excellent thing for your system to cache. You can see the current value by querying the proc filesystem again:

cat /proc/sys/vm/vfs_cache_pressure

Output
100

As it is currently configured, our system removes inode information from the cache too quickly. We can set this to a more conservative setting like 50 by typing:

sudo sysctl vm.vfs_cache_pressure=50

Output
vm.vfs_cache_pressure = 50

Again, this is only valid for our current session. We can change that by adding it to our configuration file like we did with our swappiness setting:

sudo nano /etc/sysctl.conf

At the bottom, add the line that specifies your new value:

vm.vfs_cache_pressure=50

Save and close the file when you are finished.

Conclusion

Following the steps in this guide will give you some breathing room in cases that would otherwise lead to out-of-memory exceptions. Swap space can be incredibly useful in avoiding some of these common problems.

If you are running into OOM (out of memory) errors, or if you find that your system is unable to use the applications you need, the best solution is to optimize your application configurations or upgrade your server.