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Never return to a drained laptop again!

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There have been several occasions where I haven’t used my laptop in a few days, only to return to a completely drained battery. This is not only annoying, but also not very good for the battery.

Maybe it’s a false memory, but I want to recall that most OS’s used to have a “Suspend, and then hibernate” feature. But now hibernation isn’t even a default option in Ubuntu?

This is a short guide on how to enable hibernation on Ubuntu 24.10.

Note: I’m writing this in retrospect, and the instructions are probably not complete. This is mostly as a reminder to myself if I ever have to set it up again.

Setting it up #

Resizing the swap file #

My system had a swap file (as opposed to a swap partition), but it was only 4GB in size (iirc). For hibernation purposes, it’s recommended that your swap should be the size of your RAM, although there are ways to get away with less.

This command told me that my swap file was located at /swap.img.

swapon --show

To resize it, I did the following commands:

# Disable the swap file
sudo swapoff -a
# Remove the old swap file
sudo rm /swap.img
# Use `dd` to create a new 16GB file with zeroes
sudo dd if=/dev/zero of=/swap.img count=16384 bs=1MiB
# Set the correct permissions
sudo chmod 600 /swap.img
# Enable this file to be used as swap
sudo swapon /swap.img

Instead of creating the file manually, it’s also possible to use the command mkswap, which will set up the file with the correct permissions.

Since I already had a swap file, it was already listed in /etc/fstab as:

/swap.img       none    swap    sw      0       0

Add kernel parameter to grub #

We need to pass resume and resume_offset as kernel parameters at boot.

I’m not certain this is necessary. Newer systemd and initramfs will try to figure out where the resume data is. See man initramfs-tools and man systemd-hibernate-resume. But maybe when we are using a swap file rather than a partition, we need to where on a specific partition that file is?

In /etc/default/grub, we add the resume parameter as the UUID of partition where the swap file lives. And resume_offset is where on that partition the file begins. (I think?)

How do we find these values?

The UUID we can find with the findmnt command:

sudo findmnt -no UUID -T /swap.img

And it should print the UUID. E.g.:

abcd1234-1234-1234-bdbd-asdfaaaabbbb

We find the offset with the filefrag command.

sudo filefrag -v /swap.img

File size of /swap.img is 17179869184 (4194304 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..  425983:   [22118400..]  22544383: 425984:            
...

I’ve “highlighted” where the swap file starts, our offset, with square brackets, i.e. 22118400.

GRUB_CMDLINE_LINUX_DEFAULT="<...> resume=UUID=<your uuid> resume_offset=<some offset>"

Example:

GRUB_CMDLINE_LINUX_DEFAULT="<...> resume=UUID=abcd1234-1234-1234-bdbd-asdfaaaabbbb resume_offset=1234"

And finally, to add this to your grub menu, run:

sudo update-grub

Then reboot to make it go into effect. After reboot, you can test it out by running:

systemctl hibernate

Suspend, then hibernate #

I don’t want my system to hibernate all the time, I just want it to hibernate if I don’t use it for a while. Thankfully, Chris Arderne has us covered.

We can get this behavior by changing the HandleLidSwitch parameter in /etc/systemd/logind.conf:

[Login]
HandleLidSwitch=suspend-then-hibernate

If the battery goes below 5%, the computer will hibernate. But we can also set it to hibernate based on time!

In /etc/systemd/sleep.conf we can set the HibernateDelaySec paramter. I’ve set it to go into hibernation after 30 minutes.

[sleep]
...
HibernateDelaySec=1800

And then restart the logind service:

sudo systemctl restart systemd-logind.service

Sources #