[System] Enlarging a BTRFS swap file

[steve7233]

How do I enlarge my BTRFS swap file as the kernel keeps killing steam due to low memory. My SSD is 2 TB?

[Snowhog]

AI Overview

You must disable, delete, and create a new swap file to increase its size, as there is no command to directly resize an existing btrfs swap file. First, disable the current swap file, then create a new one of the desired size, format it, and activate it.

Step-by-step instructions

Disable the swap file

Turn off the current swap file using this command:

sudo swapoff /swapfile

Delete the old swap file

Remove the old file to free up space:

sudo rm /swapfile

Create a new swap file

Create a new, larger file. Replace 1024 with your desired size in megabytes (e.g., 4096 for a 4GB file):

sudo fallocate -l 4G /swapfile

Alternatively, you can use dd:

sudo dd if=/dev/zero of=/swapfile bs=1M count=4096

Set permissions

Set the correct permissions for the new swap file:

sudo chmod 600 /swapfile

Format the new file as swap

Format the file to be used as a swap area:

sudo mkswap /swapfile

Activate the new swap file

Turn on the new swap file:

sudo swapon /swapfile

Verify the change

Check if the new swap file is active and has the correct size:

sudo swapon -s

Make the change permanent (optional but recommended)

To ensure the new swap file is mounted after a reboot, add it to your /etc/fstab file.
Open the file with a text editor: sudo nano /etc/fstab
Add the following line to the file:

/swapfile swap swap defaults 0 0

Save the file and exit the editor.

[jlittle]

My vanilla-ish 25.10 fresh install swap file was created in @swap; OP if you don't find it in /swapfile you might mount the btrfs top level and look for it there.

[oshunluvr]

Mount your root file system (I'll use /mnt). Then;

sudo swapoff -a
sudo rm /mnt/@swap/swapfile
sudo btrfs fi mkswapfile --size 4g /mnt/@swap/swapfile
sudo swapon​

The above command list assumes you have a "normal" (not custom or modified) installation.

Change 4g to something larger if you want.

[steve7233]

Mount your root file system (I'll use /mnt). Then;

sudo swapoff
sudo rm /mnt/@swap/swapfile
sudo btrfs fi mkswapfile --size 4g /mnt/@swap/swapfile
sudo swapon​

The above command list assumes you have a "normal" (not custom or modified) installation.

Change 4g to something larger if you want.

steve7233@doctortux:~$ sudo swapoff
[sudo: authenticate] Password:
swapoff: bad usage
Try 'swapoff --help' for more information.
steve7233@doctortux:~$

I tried to figure it out:

steve7233@doctortux:~$ swapoff --help

Usage:
swapoff [options] [<spec>]

Disable devices and files for paging and swapping.

Options:
-a, --all disable all swaps from /proc/swaps
-v, --verbose verbose mode

-h, --help display this help
-V, --version display version

The <spec> parameter:
-L <label> LABEL of device to be used
-U <uuid> UUID of device to be used
LABEL=<label> LABEL of device to be used
UUID=<uuid> UUID of device to be used
<device> name of device to be used
<file> name of file to be used

For more details see swapoff(8).
steve7233@doctortux:~$ sudo swapoff -a
[sudo: authenticate] Password:
steve7233@doctortux:~$ sudo rm /mnt@swap/swapfile
rm: cannot remove '/mnt@swap/swapfile': No such file or directory
steve7233@doctortux:~$ sudo btrfs fi mkswapfile --size 64g /mnt/@swap/swapfile
ERROR: cannot create new swapfile: No such file or directory
steve7233@doctortux:~$

[oshunluvr]

You left out the / on the rm command.

Why don't you do "ls" and see whats in there,

[oshunluvr]

You also didn't mount the root file system at /mnt

[steve7233]

You left out the / on the rm command.

Why don't you do "ls" and see whats in there,

steve7233@doctortux:~$ ls
DesktopDocumentsDownloadsMusicPicturesPublicTemplatesVideossnapstl
steve7233@doctortux:~$

[steve7233]

I just got up recently and my brain was firing all its neurons yet. I just realized I can't do this from the installed system. I have to use a USB stick.

[steve7233]

I don't have a 25.10 or 25.04 USB stick, but I do have a 24.04 USB stick. Can I use the 24.04 USB stick or make a 25.10 startup disk, or do I have to download the full 25.10 and make a USB stick from that?

[INVICTRA]

I just got up recently and my brain was firing all its neurons yet. I just realized I can't do this from the installed system. I have to use a USB stick.

Normally You should only need a USB stick for the classical method with a Swap Partition.
In your case you should be able to do it from your running Kubuntu. You have btrfs and a Swap File.

[steve7233]

Normally You should only need a USB stick for the classical method with a Swap Partition.
In your case you should be able to do it from your running Kubuntu. You have btrfs and a Swap File.

How does that work?

[steve7233]

I thought I had figured it out, but as you can see, /home and swap are both 21%. How do I make it permanent once I've sorted it out?
steve7233@doctortux:~$ sudo swapon
[sudo: authenticate] Password:
steve7233@doctortux:~$ df
Filesystem 1K-blocks Used Available Use% Mounted
on
tmpfs 393808 1948 391860 1% /run
/dev/sda1 1953513560 403191348 1549107932 21% /
tmpfs 1969024 517536 1451488 27% /dev/sh
m
tmpfs 5120 12 5108 1% /run/lo
ck
tmpfs 1024 0 1024 0% /run/cr
edentials/systemd-journald.service
tmpfs 1024 0 1024 0% /run/cr
edentials/systemd-resolved.service
/dev/sda1 1953513560 403191348 1549107932 21% /swap
/dev/sda1 1953513560 403191348 1549107932 21% /home
tmpfs 1969024 10124 1958900 1% /tmp
tmpfs 393804 88 393716 1% /run/us
er/1000
/dev/sda1 1953513560 403191348 1549107932 21% /mnt
steve7233@doctortux:~$

[steve7233]

I got a little further:
steve7233@doctortux:~$ sudo chmod 600 /mnt/@swap
steve7233@doctortux:~$ sudo swapon /mnt/@swap
swapon: /mnt/@swap: read swap header failed
steve7233@doctortux:~$

[INVICTRA]

Ubuntu and Kubuntu do it a bit different I think. Find out if your swapfile is at /@swap/swapfile or /swap/swapfile or deleted. Start with:

swapon --show

If no output then swapfile is already deleted. In a new Kubuntu 25.10 the file is located in /swap/swapfile
Check your fstab text file if it is the same there.


















































Turn off swap if it is still there:

sudo swapoff /swap/swapfile

Remove the file.

sudo rm /swap/swapfile

To create a new swapfile located in folder /swap/ that is mounted from the btrfs subvolume @swap for kubuntu:

sudo fallocate -l 16G /swap/swapfile

sudo chattr +C /swap/swapfile

sudo chmod 600 /swap/swapfile

sudo mkswap /swap/swapfile

sudo swapon /swap/swapfile

chattr +C is recommended to use on swapfile in btrfs filesystems.