Enlarging a BTRFS swap file

[steve7233]

NAME TYPE SIZE USED PRIO
/swap/swapfile file 64G 465.2M -2
steve7233@doctortux:~$

[steve7233]

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a device; this may
# be used with UUID= as a more robust way to name devices that works even if
# disks are added and removed. See fstab(5).
#
# <file system> <mount point> <type> <options> <dump> <pass>
UUID=9848703b-0c49-4cc1-980b-8b114cbe6fcb / btrfs subvol=/@,defaults,noatime,autodefrag,discard,compress=lzo 0 0
UUID=9848703b-0c49-4cc1-980b-8b114cbe6fcb /home btrfs subvol=/@home,defaults,noatime,autodefrag,discard,compress =lzo 0 0
UUID=9848703b-0c49-4cc1-980b-8b114cbe6fcb /swap btrfs subvol=/@swap,defaults 0 0
/swap/swapfile swap swap defaults 0 0
tmpfs /tmp tmpfs defaults,noatime,mode=1777 0 0

[steve7233]

Looks like it shows the one I tried to create, but I can't get it working yet.

[Snowhog]

Have you read the documentation? Swapfile - BTRFS documentation - Swapfile

[steve7233]

When I try to use sudo swapon /swap/swapfile, it says device or resource busy. How do I find out what is using it?

[steve7233]

I accidentally got it working. I don't know how. I will figure out how to make it permanent later. Oops, I forgot to click the post button. I had typed this a few hours ago, but forgot to post it. I just got back, and now I can see about making it permanent.

[steve7233]

Should I replace the swapfile line in my fstab, or will the old one work? See comment #17 for my fstab.

[INVICTRA]

Should I replace the swapfile line in my fstab, or will the old one work? See comment #17 for my fstab.

I would leave it. It looks OK.

[Snowhog]

UUID=9848703b-0c49-4cc1-980b-8b114cbe6fcb /swap btrfs subvol=/@swap,defaults 0 0
/swap/swapfile swap swap defaults 0 0

You have a subvolume for swap and a swapfile? Why! And, the mount point you have in the /swap/swapfile stanza isn't correct (swap); it should be none.

AI Overview

An /etc/fstab entry for a btrfs swapfile should have the following format: /path/to/swapfile none swap defaults 0 0. The none in the second field indicates that the swapfile is not a typical mount point, and the third field specifies the filesystem type as swap. The defaults and 0 0 are standard options for swapfiles, ensuring they are handled correctly at boot.

Device	/swapfile	The absolute path to the swapfile. You can also use /dev//swapfile` if the file is in the root.
Mount Point	none	This is the mount point. For swap files, this should be none.
Filesystem Type	swap	This specifies the filesystem type as swap.
Options	defaults	This is a standard option for swap files. You may also see sw.
Dump	0	This field is not used for swap files and should be 0.
Pass	0	This field is not used for swap files and should be 0.

Example entry for a swapfile

/swapfile none swap defaults 0 0

How to apply the entry

• Create the swapfile: Use dd or fallocate to create a swapfile of the desired size.
• Set correct permissions:
  chmod 600 /swapfile
• Set up the swap file:
  mkswap /swapfile`
• Add the entry to /etc/fstab:
  echo '/swapfile none swap defaults 0 0' | sudo tee -a /etc/fstab
• Activate the swapfile:
  sudo swapon -a

[INVICTRA]

You have a subvolume for swap and a swapfile? Why! And, the mount point you have in the /swap/swapfile stanza isn't correct (swap); it should be none.

I have seen that Kubuntu does it this way in 25.10 on a fresh installation. It seems to solve some auto-mount problems with btrfs for some users.

AI Overview

Mount point	none	swap
Type	swap	swap
Effect	Swapon called, standard	Swapon called, systemd ordering safer for Btrfs

So essentially, both work. Ubuntu switched to the swap mount point as a practical workaround for Btrfs + systemd startup ordering.