504. Swap file and hibernation on debian

I've got an SSD and 4 gb ram on my laptop, and have no desire to use to actual swap during normal use. However, I'd like to be able to let my laptop go into hibernation -- even suspend seems to be draining my battery pretty fast (at maybe a quarter of the rate of keeping the laptop on).

Does it make sense hibernating a laptop with SSD i.e. one which boots in ten seconds flat? Probably not. But we humans are greedy by nature.

So I need to set up a swap file, disable swapping, and see if I can use it for hibernation.

As usual, the best source of information is the archlinux wiki.
https://wiki.archlinux.org/index.php/Swap#Swap_file
https://wiki.archlinux.org/index.php/Pm-utils#Using_Swap_file_instead_of_regular_swap_partition

Anyway, having made it work and tried it out I would say this is really not worth the hassle IF you have an SSD. 

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1. The swap file
This is my file system layout:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1        39G   20G   18G  53% /
udev             10M     0   10M   0% /dev
tmpfs           380M  816K  380M   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           760M  1.1M  759M   1% /run/shm
/dev/sda2       109G   71G   33G  69% /home

In my case I'd say that / has enough space to handle a swap file. My RAM is '4 gb' (really 3.71 Gb) -- according to this and this my swap should equal my RAM, which makes intuitive sense.

Since 4,000,000,000 byte is 3.76 gb, and 4000*1024*1024 (i.e. 4000M) is 3.91 Gb, I think 4000M should be ok:

su -
fallocate -l 4000M /swapfile
chmod 600 /swapfile
mkswap /swapfile

Setting up swapspace version 1, size = 4095996 KiB
no label, UUID=2a8de3d1-14f6-473f-b40f-31618fd81169

echo 'vm.swappiness=1' >> /etc/sysctl.d/50-local.conf
echo '/swapfile none swap defaults 0' >> /etc/fstab

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2. "PM: Swap header not found"

To try it out without rebooting:

sudo sysctl -w vm.swappiness=1
sudo swapon /swapfile
pm-is-supported --hibernate 
echo $?

0

If you got 0, then you're good to go.
sudo pm-hibernate

Trying it out the first time I got "PM: swap header not found" and some weird behaviour. This has been mentioned e.g. here. A step-by-step guide is here: https://ubuntuforums.org/showthread.php?t=1042946

Get the UUID of the partition on which the swapfile is located:

mount | grep " / "

/dev/disk/by-uuid/8adf424c-c375-4035-8d5d-181489b4461b on / type ext4 (rw,noatime,nodiratime,discard,errors=remount-ro,data=ordered)

sudo filefrag -v /swapfile | grep "First block:"

The latter command gave nothing, so I then did:

sudo filefrag -v /swapfile|less

Filesystem type is: ef53
File size of /swapfile is 4194304000 (1024000 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       0:    7182336..   7182336:      1:            
   1:        1..    6143:    7182337..   7188479:   6143:             unwritten
   2:     6144..    8191:    7190528..   7192575:   2048:    7188480: unwritten

So now we have the UUID (8adf424c-c375-4035-8d5d-181489b4461b) and the offset (7182336).

su -
echo "resume=UUID=8adf424c-c375-4035-8d5d-181489b4461b resume_offset=7182336" | sudo tee /etc/initramfs-tools/conf.d/resume
exit

Edit /etc/default/grub and add the same line to GRUB_CMDLINE_LINUX_DEFAULT:

GRUB_CMDLINE_LINUX_DEFAULT="quiet resume=UUID=8adf424c-c375-4035-8d5d-181489b4461b resume_offset=7182336"

Run

sudo update-grub
sudo update-initramfs -u

Reboot.

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3. Hibernating.

sudo pm-hibernate

On my lenovo sl410 what I see is the screen go blank save for a blinking "-", and the little crescent light at the front of the laptop starting to blink (next to the battery light).

After 5-10 seconds the laptop turns off.

Hitting the power button starts up the laptop -- you get the bios screen, then the grub menu, and at this point you're thinking that the whole exercise has failed -- but it ends up booting into the same situation as you had when you did pm-hibernate i.e. it worked. I mean, this is how hibernation works -- but I had honestly never used anything other than suspend before, so...well..there's a first time for everything.

Overall, on a laptop with an SSD, resuming from hibernation is about as slow as a cold start -- with entering hibernation taking longer than a shutdown. On a laptop with a spinning disk this could presumably be worth it.