This is a step-by-step tutorial on how to create or remove a swapfile on a Linux machine, and whether you should create swap space in the first place.

Also included is the most correct fstab entry: most articles get this “wrong” (but it doesn’t actually matter).

Should you use swap?

A common misconception is that adding swap to your system can reduce performance. However, even if you aren’t running out of RAM, it can still be beneficial to add swap.

See these points according to an article by Hayden James:

• Even if there is still available RAM, the Linux Kernel will move memory pages that are hardly ever used into swap space.

• It’s better to swap out memory pages that have been inactive for a while, keeping often-used data in cache, and this should happen when the server is most idle, which is the aim of the Kernel.

• Avoid setting your swap space too large if it will result in prolonging performance issues, outages, or your response time (without proper monitoring/alerts).

Should you use a swap file or partition?

Swap partitions should be preferred because swapfiles tend to be slower and slightly more complex to set up, especially if hibernating. A swapfile might be preferred due to its flexibility (ease to resize), but if you use LVM then you can easily resize the swap partition anyways.

Tutorial

Step 1: Create the file

The first step is to allocate the file.

console

1

# dd if=/dev/zero of=/swapfile bs=1M count=[size in MiB] status=progress

Replace [size in MiB] with the size of your swapfile in Mebibytes (MiB). You can use this online converter to convert from Gibibytes (GiB, often confused with Gigabytes, but that’s a misconception for another time) to MiB, which you can put in the command.

Or you can look at this table for common sizes:

If you aren’t sure how big your swapfile should be, take a look at Table 15.1 on this Red Hat documentation page. Remember that if your first swapfile isn’t large enough, you can create another one.

On my computer I have 32 GiB of RAM and a 16 GiB swap partition, rather than swapfile, because I can flexibly expand the swap partition using LVM.

Step 2: Change swapfile permissions

The swapfile should only be readable by the system (root user). Run this command to change it:

console

1

# chmod 600 /swapfile

Step 3: Format the swapfile

Use the mkswap command to format the file to be used as swap (basically just add a header to identify it):

console

1
2
3

# mkswap /swapfile
Setting up swapspace version 1, size = 4 GiB (4294963200 bytes)
no label, UUID=a0b87eca-b951-4344-be2d-020d77cdef48

Step 4: Create an entry in /etc/fstab

An entry needs to be added to /etc/fstab for the swapfile to be enabled during bootup. Open /etc/fstab in a text editor, and add this line to the end (add spaces to line up with other entries if you want, whitespace is ignored):

text

1

/swapfile none swap sw 0 0

Step 5: Enable the swapfile

Adding the fstab entry won’t enable the swapfile until a reboot. To enable it now, use the swapon command.

console

1

# swapon /swapfile

Finally: check the swap status

Use swapon and free to verify that your new swapfile has been added:

console

1
2
3
4
5
6
7
8

$ swapon --show
NAME       TYPE SIZE USED PRIO
/swapfile  file   4G   0B   -2

$ free -h
               total        used        free      shared  buff/cache   available
Mem:            31Gi       3.6Gi        20Gi       233Mi       6.9Gi        26Gi
Swap:          4.0Gi          0B       4.0Gi

Removing a swapfile

Step 1: Disable/unload the swapfile

console

1
2
3
4
5
6
7

$ swapon --show
NAME       TYPE SIZE USED PRIO
/swapfile  file   4G   0B   -2

$ # the path to the swapfile I want to remove is `/swapfile`

# swapoff /swapfile

Step 2: Remove the entry from fstab

Open /etc/fstab in a text editor and find the line matching the swapfile you want to remove, and delete it. For example:

text

1

/swapfile none swap sw 0 0

Step 3: Delete the actual file

console

1

# rm /swapfile

References

Use dd instead of fallocate

See “Files with holes” under the man page.

The options field in fstab

On Linux, sw doesn’t mean anything for swapon, so it gets ignored. See:

• StackExchange answer 1
• StackExchange answer 2
• parse_options in the swapon source code
• options set by fstab defaults (none have any effect on swapon)