This guide is mostly a copy of the guide for Fedora 36 on the Fedora Magazine.
(all commands as root)
Create a place to put it.
btrfs subvolume create /swap
To create the file, you need to know how much RAM you have and use. You need to be able to store the contents of the RAM + your zram drive.
Fedora Magazine's exact quote is:
In this example the machine has 16G of RAM and a 8G zram device. ZRAM stores roughly double the amount of system RAM compressed in a portion of your RAM. Let that sink in for a moment. This means that in total the memory of this machine can hold 8G * 2 + 8G of RAM which equals 24G uncompressed data. Create and configure the swapfile using the following commands.
Find ZRAM:
swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 8G 0B 100
In my case, for 32GB of RAM and 8GB of zram I used 38GB. I used the old and tried RAM + sqrt(RAM), and not what Fedora Mag said, and I may have done this wrong. My understanding is this: ZRAM stores double the amount of RAM allocated. So 8+8GB = 16GB. And then you got the left over from your RAM. In my case, 32-8=24GB, so the total should be 16+24=40GB. Probably this is better and I should change it from 38GB. It's not like I'm saving much anyway.
Create the file.
touch /swap/swapfile
# Disable Copy On Write on the file
chattr +C /swap/swapfile
fallocate --length 40G /swap/swapfile
chmod 600 /swap/swapfile
mkswap /swap/swapfile
To keep the swap always on:
Add it to your /etc/fstab
/swap/swapfile swap swap defaults 0 0
Note: Only needed if you plan to move to zswap.
You now need the module resume in initramfs.
cat <<-EOF | sudo tee /etc/dracut.conf.d/resume.conf
add_dracutmodules+=" resume "
EOF
dracut -f
Now we need a kernel option for when we reboot from hibernation, so that the system knows where to read the contents of the swapfile. For that we need the UUID of the partition where the file is located on.
findmnt -no UUID -T /swap/swapfile
b6b8fa59-92cc-4d03-8d8f-d66dab76d433
And the offset. This is the tricky part for BTRFS. Download the source of the btrfs_map_physical tool (open the link, download from git, don't just right click-save file) and compile it:
Run gcc where the downloaded file is:
gcc -O2 -o btrfs_map_physical btrfs_map_physical.c
Then execute the compiled file. Make your terminal as wide as possible for this.
./btrfs_map_physical /swap/swapfile
You need the last number from the first line with numbers, the PHYSICAL OFFSET. The output can be mingled so make sure you copy the number correctly.
0 4096 0 regular 268435456 60255100928 268435456 1 60263489536
In my case it's 60263489536.
Find the pagesize with
getconf PAGESIZE
Mine is 4096, so the offset calculation is 60263489536/4096=14712766.
If you use grub you need to add the kernel option with grubby as so:
grubby --args="resume=UUID=b6b8fa59-92cc-4d03-8d8f-d66dab76d433
resume_offset=14712766" --update-kernel=ALL
If you use systemd-boot, currently (F39) you need to add this line in all loaders. Hopefully with the next kernel install, the system will use the default for grub to write the new loaders with the correct options.
You need to only use the swapfile for hibernation, not swap, so two services will do that.
cat <<-EOF | sudo tee /etc/systemd/system/hibernate-preparation.service
[Unit]
Description=Enable swap file and disable zram before hibernate
Before=systemd-hibernate.service
[Service]
User=root
Type=oneshot
ExecStart=/bin/bash -c "/usr/sbin/swapon /swap/swapfile && /usr/sbin/swapoff /dev/zram0"
[Install]
WantedBy=systemd-hibernate.service
EOF
and
systemctl enable hibernate-preparation.service
Then,
cat <<-EOF | sudo tee /etc/systemd/system/hibernate-resume.service
[Unit]
Description=Disable swap after resuming from hibernation
After=hibernate.target
[Service]
User=root
Type=oneshot
ExecStart=/usr/sbin/swapoff /swap/swapfile
[Install]
WantedBy=hibernate.target
EOF
and
systemctl enable hibernate-resume.service
Systemd does memory checks on login and hibernation. In order to avoid issues when moving the memory back and forth between swapfile and zram disable some of them.
mkdir -p /etc/systemd/system/systemd-logind.service.d/
cat <<-EOF | sudo tee /etc/systemd/system/systemd-logind.service.d/override.conf
[Service]
Environment=SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1
EOF
mkdir -p /etc/systemd/system/systemd-hibernate.service.d/
cat <<-EOF | sudo tee /etc/systemd/system/systemd-hibernate.service.d/override.conf
[Service]
Environment=SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1
EOF
Reboot.
Try to hibernate (as a user, not root).
systemctl hibernate
The following command will fail, returning you to a login prompt.
After you’ve logged in again check the audit log, compile a policy and install it. The -b option filters for audit log entries from last boot. The -M option compiles all filtered rules into a module, which is then installed using semodule -i.
As root,
audit2allow -b
#============= systemd_sleep_t ==============
allow systemd_sleep_t unlabeled_t:dir search;
cd /tmp
audit2allow -b -M systemd_sleep
semodule -i systemd_sleep.pp
UPDATE I had to do the Selinux step after I upgraded from F39 to F40
Check that hibernation is working via systemctl hibernate again.
systemctl hibernate
That should now work.
Check that only ZRAM appears here:
swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 8G 0B 100
All is good.
The old(er) function of suspend-then hibernate is gone.
Previously, when selected, the system would suspend after some time of inactivity, or with the lid down, then after some time it would hibernate. A great feature that worked well.
However, this is no more. The systemd devs have decided this is not a good idea, and now suspend-then-hibernate is totally irrelevant. The system suspends as before, but only goes to hibernate if the battery is at 5% or lower. It will no longer hibernate after some time (given with HibernateDelaySec=3600 as shown below).
This is completely idiotic and useless. Hybrid sleep was available to avoid data loss in case of battery drain. With Hybrid Sleep, the system would dump the RAM to disk (hibernate, aka S4) and go to suspend. If the power died, it would resume from hibernate as per normal. This is still available, but sadly suspend-then-hibernate is not.
See this (now locked) thread here. It is clear the devs (bluca) are not going to listen. Shame!
Update it's been reinstated, see below.
Now what happens is that when the system goes to hibernation, the RAM is written on the swap. The system reserves an amount on the swap for this, typically 2/5 of the RAM.
You can check how much that is with:
cat /sys/power/image_size
6871947674
You can do the calculation, but 6871947674 is 6GiB, which is 2/5 of 16GB. In order of the RAM to fit that, it needs to be compressed.
This process adds time when going to hibernation and when resuming. I have not measured the impact, so this whole excercise may be pointless.
This solution, however, means you can hibernate using a swap file/partition smaller than your RAM.
In any event, if the swap file/partition is large, and I'm using 24GB, you don't need to do that. You can just dump the RAM on to the swap file/partition. And you don't need to compress it for that.
Note: My understanding is limited.
I'm following this arch wiki article and its links. I also use this post
So, I have changed from 2/5 my RAM to all my RAM, which is 16GiB or 17179869184 bytes (multiply by 1024 3 times). To do this, create a file.
cat /etc/tmpfiles.d/hibernation_image_size.conf
# Path Mode UID GID Age Argument
w /sys/power/image_size - - - - 17179869184
I would think this is enough to know so that no compression is used, however I also added hibernate=nocompress as a kernel option.
Maybe it's placebo, but the system certainly goes to hibernation quicker. Resuming is still slow (after all 16GiB needs to be read from the disk). With a fast NVME PCIe 4.0 drive (I use a 990Pro, ~7GiB/s) this should take around 3 seconds. It takes a bit more, but it's fine.
This guide is better. Not that this matters anymore as the suspend-then hibernate is no longer important (see above).
You can add the extension to make the hibernation option appear in the power menu. It's Hibernation Status Button.
(this part is copied from the top comment here
Finally you can set the default behaviour when you close the lid, or set the default sleep option to sleep-then-hibernate.
Edit /etc/systemd/sleep.conf
UPDATE Since systemd 254, you need to also have
And change the line: SuspendEstimationSec=10min. This overcomes the regression (and reinstatement) of sleep-then-hibernte where it only worked if the battery was under 5%. The new addition tells the system how offen to check the battery, for this new feature to work.
That is, if the battery falls to 5% before hibernation is set to start, the system still goes to hibernation. I'll add the link later.
[Sleep]
HibernateDelaySec=3600s
SuspendEstimationSec=10min
This configures the time before it goes to hibernation from sleep!
Test with:
sudo systemctl suspend-then-hibernate
Finally, you can set the default behaviour on closing the lid by editing /etc/systemd/logind.conf.
You need to find option HandleLidSwitch=, uncomment it and change to HandleLidSwitch=suspend-then-hibernate.
Then you need to restart systemd-logind service (warning! you user session will be restarted) by the next command:
sudo systemctl restart systemd-logind.service
That's all! Now you can use this nice function.