Hibernating in Debian using a temporary swap file

This article explains how to configure a Debian Jessie system to hibernate using swap space that’s created just before suspending and destroyed just after resuming. This trick allows hibernation on a swap-free or swap-lite system—useful on a desktop system running with enough RAM to make swap space otherwise unnecessary.

How to make it work

Determine the root partition of your system

$ rootdev=$(mount | grep ' / ' | awk '{ print $1 }')

Important! Make sure you get this right. Otherwise your system may fail to suspend or—much worse—fail to resume.

$ echo $rootdev
/dev/sda1

The second line (e.g., /dev/sda1) should be your system’s root partition.

Install and configure uswsusp

$ sudo apt-get install uswsusp &&
  sudo sh -c "cat <<EOF >/etc/uswsusp.conf
resume device = $rootdev
compress = y
early writeout = y
shutdown method = platform
image size = 0
resume offset = 0
EOF"

Hibernating using a swap file—as opposed to a swap partition—requires special support. We’ll use uswsusp.

The /etc/uswsusp.conf is the configuration file for uswsusp. The image size and resume offset lines are placeholders that we’ll overwrite each time before suspending.

Note. It’s also possible to hibernate to a swap file using systemd, but my only experience doing this is with Arch Linux, and, as far as I know, the swap file must exist upon boot-up, thus preventing the swap file from being created on demand.

Update Grub to add the resume kernel parameter

$ { grep -q '\<GRUB_CMDLINE_LINUX_DEFAULT=.*resume=' /etc/default/grub &&
      (echo 'Warning: The `resume` parameter is already set.' >&2 || true) } ||
    { sudo sed --in-place \
        -e "s#^\\(GRUB_CMDLINE_LINUX_DEFAULT=\"\\)#\\1resume=$rootdev #" \
        /etc/default/grub &&
      sudo update-grub; }

The resume kernel is necessary to resume from a swap file. Otherwise the kernel will not find the swap file, and the system will boot normally—as though rebooting.

If the resume parameter is already set, you’ll need to take intelligent action to resolve.

Install the dynamic-hibernate script

$ sudo wget -O /usr/local/sbin/dynamic-hibernate \
    https://raw.githubusercontent.com/cmbrandenburg/pcconf/e7dbe6128a9267b1f55fed27b22fcbfdc1734b3e/bin/dynamic-hibernate &&
  sudo chmod +x /usr/local/sbin/dynamic-hibernate

The dynamic-hibernate script does the actual hibernation. It:

1. Sets up a temporary swap file,
2. Updates the /etc/uswsusp.conf with the location of the new swap file,
3. Re-creates the system’s initial RAM disk,
4. Suspends the system, and
5. Deletes the swap file after resuming.

By default, the script requests the kernel to store the smallest image possible when suspending, presumably by discarding disk buffers and disk-backed pages that may be safely reloaded after resuming. In my experience, smaller images yield faster resume speeds when using a spinning disk, and SSDs are fast enough to make any difference negligible. Nevertheless, if you want to change this behavior then modify the image size value in the script.

Give yourself sudo permission to hibernate (optional)

$ sudo sed --in-place \
  -e "\$a$USER ALL = NOPASSWD: /usr/local/sbin/dynamic-hibernate" \
  /etc/sudoers

This gives permission to your current user account to run the dynamic-hibernate script without entering a password.

Create a script to lock the screen and hibernate (optional)

$ mkdir -p ~/bin &&
  touch ~/bin/hibernate &&
  chmod +x ~/bin/hibernate &&
  cat <<EOF >~/bin/hibernate
#!/bin/sh
gnome-screensaver-command --lock
sudo /usr/local/sbin/dynamic-hibernate
EOF

Locking the screen requires the user to enter a password after resuming. Without locking the screen, anyone who resumes your computer during hibernation will gain access to your system.

The above script works only if you’re using the Gnome screensaver. If you’re using a different screensaver then you must change the ~/bin/hibernate script appropriately.

Verify everything works

$ ~/bin/hibernate

Or:

$ sudo /usr/local/sbin/dynamic-hibernate

Your system will take a few moments to save state before powering off. Power the system back on, and the system should restore its state as it was before suspending.

Additional considerations

Once you get hibernation working, it should continue to work reliably. However, here are some further considerations.

• Don’t hibernate after upgrading the kernel. Reboot first. Otherwise the kernel you resume with will be a different kernel than what you suspended with, and the mismatch will cause mayhem.

• If your system fails to suspend then there’s a good chance the hibernation script will print a useful error message.

• If your system fails to suspend or resume then you may need to manually delete the swap file that gets left behind (/swap.hibernate).

• If your system fails to resume then your system will suffer the same consequences as though it crashed. This includes loss of application state, file corruption, etc.

References

1. Computer setup notes: These are the notes I use to set up a desktop system.

Got feedback? Email me at c.m.brandenburg@gmail.com.