Suspend to RAM, usually called suspend, cuts power to most parts of the machine aside from the RAM, which is required to restore the machine's state. Because of the large power savings, it is advisable for laptops to automatically enter this mode when the computer is running on batteries and the lid is closed (or the user is inactive for some time).
Suspend to disk, usually called hibernate, saves the machine's state into swap space and completely powers off the machine. When the machine is powered on, the state is restored. Until then, there is zero power consumption.
Suspend to both, usually called hybrid suspend, saves the machine's state into swap space, but does not power off the machine. Instead, it invokes usual suspend to RAM. Therefore, if the battery is not depleted, the system can resume from RAM. If the battery is depleted, the system can be resumed from disk, which is much slower than resuming from RAM, but the machine's state has not been lost.
The most straightforward approach is to directly inform the in-kernel software suspend code (swsusp) to enter a suspended state; the exact method and state depends on the level of hardware support. On modern kernels, writing appropriate strings to /sys/power/state is the primary mechanism to trigger this suspend.
Userspace Software Suspend
systemd provides native commands for suspend, hibernate, hybrid suspend
The kernel documentation of Power Management Interface for System Sleep is here
/sys/power/state is the system sleep state control file. In it listed a list of supported sleep states, encoded as:
sys/power/disk controls the operating mode of hibernation, or suspend to disk. It tells the kernel what to do next after the creation of hibernation image. Currently selected option is printed in square brackets.
/sys/power/image_size controls the size of hibernation images.
It can be written a string representing a non-negative integer that will be used as a best-effort upper limit of the image size, in bytes. The hibernation core will do its best to ensure that the image size will not exceed that number. However, if that turns out to be impossible to achieve, a hibernation image will still be created and its size will be as small as possible. In particular, writing '0' to this file will enforce hibernation images to be as small as possible.
Reading from this file returns the current image size limit, which is set to around 2/5 of available RAM by default.
/sys/power/pm_trace and more information is here)
A swap partition or swap file is need. Then you will point the kernel to the swap using the resume= kernel parameter in oot loader. To configure the initramfs is also needed to tell the kernel.
According to previous content of system sleep, a small swap partation is also very likely to hibernate successfully. By the way, you are strongly recommended to read the kernel documentation mentioned previously, which offers numerous help in a straightforward way.
The size of swap_file or swap_partition is recommended to be about one to twice the RAM size.
According to kernel documentation, to use a swap_file, you need to:
bash
dd if=/dev/zero of=<path_to_swap_file> bs=1024 count=<swap_size_in_k>
mkswap <path_to_swap_file>
swapon <path_to_swap_file>
Use an application that will bmap the swap_file with the help of the FIBMAP ioctl and determine the location of the file's swap header, as the offset, in PAGE_SIZE units, from the beginning of the partition which holds the swap file.
Update kernel parameter
resume=<swap_file_partition> resume_offset=<swap_file_offset>
where swap_file_partition is the partition on which the swap file is located, and swap_file_offset is the offset of the swap header determined by the application in step 2.
swap_file:
shell
findmnt -no UUID -T <path_to_swap_file>
swap_file_offset:
shell
filefrag -v <path_to_swap_file> | awk '{if($1="0:"){print substr($4, 1, length($4)-2)} }'
resume hook should be added in /etc/mkinitcpio.conf after base and udev.
If systemd hook is used, the hibernation mechanism is already provided, and no further hooks are needed.
resume=swap_device
Here lists several boot loaders for demonstration. You can get PARTUUID using blkid
edit /boot/oader/entries/arch.conf
options root=UUID=... rw resume=PARTUUID=swap_area
edit /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="resume=PARTUUID=swap_area"
then regenerate the grub.cfg:
grub-mkconfig -o /boot/grub/grub.cfg
same
resume=PARTUUID=swap_area
Obtain the volume's major and minor device numbers using lsblk, and echo them in format major:minor to /sys/power/resume.
echo <MAJ>:<MIN> > /sys/power/resume
If using a swap file, additionally echo the resume offset to /sys/power/resume_offset
echo <MAJ>:<MIN> > /sys/power/resume
echo <swap_file_offset> > /sys/power/resume_offsets