====== Hibernate from a non activate swapfile from btrfs ======

*** Important! *** This didn't work! It is not secure! When wiping happens, the file gets moved to another place by btrfs, then, this doesn't work for two reasons: 1) Wiping happens in other parts of the disk, and the offset changes and has to be updated in all configurations files after wiping everytime.

* This was just a stupid experiment.

===== Create swapfile =====


  * Create a separate swap volume (not necessary)

  sudo btrfs subvolume create /mnt/btrfsbase/swap
  
  * Add it to /etc/fstab


  UUID=8f9ede3a-912f-40d6-9335-90ed3186e3e4   		      	    /swap	      	     btrfs defaults,noautodefrag,nodatacow,nodiscard,noatime,subvol=swap	0 0

  * Be sure there is no discard nor autodefrag options activated for this subvolume

  * Create a swapfile

  cd /swap
  sudo truncate -s 0 swapfile
  sudo chattr +C swapfile
  btrfs property set swapfile compression none
  sudo chmod 600 swapfile
  sudo dd if=/dev/zero of=swapfile bs=1M count=24576 #reserve 24GB
  sudo mkswap /swap/swapfile

===== Find out resume device and offset =====

  * Find out resume device:

  sudo findmnt -no UUID -T /swap/swapfile

  * swap_offset command doesn't work correctly with btrfs. Download this https://github.com/osandov/osandov-linux/blob/master/scripts/btrfs_map_physical.c and compile it:

  gcc -O2 -o btrfs_map_physical btrfs_map_physical.c

  * Calculate offset
  * Write down the last number from the output of the following command:

  sudo btrfs_map_physical /swap/swapfile | head -n 2

  * Result:

  FILE OFFSET	FILE SIZE	EXTENT OFFSET	EXTENT TYPE	LOGICAL SIZE	LOGICAL OFFSET	PHYSICAL SIZE	DEVID	PHYSICAL OFFSET
  0	134217728	0	regular	134217728	4594864128	134217728	1	4594864128

  * Write down the output of the following command:

  sudo getconf PAGESIZE

  Result: 4096

  * Calculate offset: Divide the first number under the PAGESIZE: offset = PHYSICAL_OFFSET/PAGESIZE

4594864128/4096=1121793


===== Configure system for hibernation =====


  * Create /etc/initramfs-tools/conf.d/resume and put the resume and offset data on it:

  RESUME=UUID=8f9ede3a-912f-40d6-9335-90ed3186e3e4 resume_offset=1121793

  * Modify /etc/default/grub with the corresponding resume and offset values:

  GRUB_CMDLINE_LINUX_DEFAULT="resume=UUID=8f9ede3a-912f-40d6-9335-90ed3186e3e4 resume_offset=1121793 quiet btusb.enable_autosuspend=n"

  * Activate the swap file

  sudo swapon /swap/swapfile

  * Install uswsusp package
  * Edit /etc/uswsusp.conf:

  resume device = /dev/disk/by-uuid/8f9ede3a-912f-40d6-9335-90ed3186e3e4
  early writeout = y
  image size = 0
  RSA key file = /etc/uswsusp.key
  shutdown method = platform
  resume offset = 1121793

  * Activate a systemd service:

  sudo systemctl edit systemd-hibernate.service

  * With:

  [Service]
  ExecStart=
  ExecStartPre=-/bin/run-parts -v -a pre /lib/systemd/system-sleep
  ExecStart=/usr/sbin/s2disk
  ExecStartPost=-/bin/run-parts -v --reverse -a post /lib/systemd/system-sleep

  * Update initramfs and grub

  sudo update-grub
  sudo update-initramfs -k all -u
  sudo update-grub

  * Test the hibernation:

  sudo s2disk

I have a system with encrypted swap, but I don't want to have encryption on the hibernation swap file. My idea is to use hibernation only when strictly necessary, because I don't want to need to put a password during booting.  Then, this swap file should actually be always deactivated except before hibernation. When resuming, this swap space should be deactivated and then securely wiped! The script to do this would be this: my_server_hibernate.sh:

#!/bin/bash

  echo "Activating swap from swapfile"
  sudo swapon /swap/swapfile
  sleep 1
  echo "Hibernating"
  sleep 0.2
  xtrlock -b &
  sudo s2disk
  echo "Returning from Hibernate"
  sleep 10
  echo "Deactivating swapfile"
  sudo swapoff /swap/swapfile
  sleep 5
  echo "Wiping swapfile"
  #sudo wipe -k -f -q -Q 1 /swap/swapfile
  sudo dd if=/dev/zero of=/swap/swapfile bs=1M count=24576 status=progress #reserve 24GB
  sync
  sleep 1
  echo "Making swapfs again"
  sudo mkswap /swap/swapfile
  echo "All done, ready to hiberante again"

===== References =====


https://wiki.archlinux.org/index.php/Power_management/Suspend_and_hibernate#Hibernation_into_swap_file_on_Btrfs

https://wiki.archlinux.org/index.php/Power_management/Suspend_and_hibernate#Hibernation_into_swap_file

https://wiki.archlinux.org/index.php/Swap#Swap_file_creationhttps://wiki.archlinux.org/index.php/Swap#Swap_file_creation

https://askubuntu.com/questions/6769/hibernate-and-resume-from-a-swap-file

https://wiki.debian.org/Hibernation/Hibernate_Without_Swap_Partition

https://wiki.debian.org/Hibernation

https://unix.stackexchange.com/questions/521686/using-swapfile-for-hibernation-with-btrfs-and-kernel-5-0-16-gentoohttps://unix.stackexchange.com/questions/521686/using-swapfile-for-hibernation-with-btrfs-and-kernel-5-0-16-gentoo