Linux add swap fstab

How To Add Swap Space on Ubuntu 20.04

One way to guard 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 an Ubuntu 20.04 server.

What is Swap?

Swap is a portion of hard drive storage that has been set aside for the operating system to temporarily store data that it can no longer hold in RAM. This lets you 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:

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:

Output
 total used free shared buff/cache available Mem: 981Mi 122Mi 647Mi 0.0Ki 211Mi 714Mi 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:

Output
Filesystem Size Used Avail Use% Mounted on udev 474M 0 474M 0% /dev tmpfs 99M 932K 98M 1% /run /dev/vda1 25G 1.4G 23G 7% / tmpfs 491M 0 491M 0% /dev/shm tmpfs 5.0M 0 5.0M 0% /run/lock tmpfs 491M 0 491M 0% /sys/fs/cgroup /dev/vda15 105M 3.9M 101M 4% /boot/efi /dev/loop0 55M 55M 0 100% /snap/core18/1705 /dev/loop1 69M 69M 0 100% /snap/lxd/14804 /dev/loop2 28M 28M 0 100% /snap/snapd/7264 tmpfs 99M 0 99M 0% /run/user/1000 

The device with / in the Mounted on column is our disk in this case. We have plenty of space available in this example (only 1.4G 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 create a swap file on our filesystem. We will allocate a file of the size that we want called swapfile in our root ( / ) directory.

The best way of creating a swap file is with the fallocate program. This command instantly creates a file of the specified size.

Since the server in our example has 1G of RAM, we will create a 1G file in this guide. Adjust this to meet the needs of your own server:

We can verify that the correct amount of space was reserved by typing:

Our file has been 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 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:

Verify the permissions change by typing:

Output
-rw------- 1 root root 1.0G Apr 25 11:14 /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:

Output
Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes) no label, UUID=6e965805-2ab9-450f-aed6-577e74089dbf 

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

Verify that the swap is available by typing:

Output
NAME TYPE SIZE USED PRIO /swapfile file 1024M 0B -2 

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

Output
 total used free shared buff/cache available Mem: 981Mi 123Mi 644Mi 0.0Ki 213Mi 714Mi Swap: 1.0Gi 0B 1.0Gi 

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:

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

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:

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:

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:

At the bottom, you can add:

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:

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:

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:

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

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.

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Adding a new swap file. How to edit fstab to enable swap after reboot?

I’ve deleted my existing swap partition due to some partitioning problem. I don’t have a swap space now so I’ve created a swap file with:

dd if=/dev/zero of=/root/myswapfile bs=1M count=1024 

/root/myswapfile file 1048572 1320 -1 

Swap on SSD is dangerous cause SSD drives have low duration on intense write operations. This means you can really consume your precious and expensive SSD drive like that.

Notice that bs=1G count=1 would give the same result, have a look at the dd manpage for the other units.

1 Answer 1

These ares the steps to create a swap on a file:

Create a large file e.g. with

sudo mkdir -p /var/cache/swap/ # create a directory that holds the swap file sudo dd if=/dev/zero of=/var/cache/swap/myswap bs=1M count=4096 # for 4 GByte 

Announce swap to the system

sudo chmod 0600 /var/cache/swap/myswap # only root should have access sudo mkswap /var/cache/swap/myswap # format as swap sudo swapon /var/cache/swap/myswap # announce to system 

Insert the following line in /etc/fstab for swap from the next boot:

/var/cache/swap/myswap none swap sw 0 0 

Note: In case you have your system files on a SSD you may want to consider to hold your swap file on a hard disk location.

Also note: You can not use a swap file for hibernation (see Ubuntu SwapFaq)

Additional note for Ubuntu >= 17.04: A swap on file /swapfile is created by default in a new installation (when no swap partition was present). We can manually create a swap partition later if we prefer.

In case we want to replace an existing swap (e.g. partition) with another swap (e.g. on file) we need to remove the old swap with

Then remove the swap entry from /etc/fstab or replace it with the new swap respectively.

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