As seen on https://twitter.com/nabijaczleweli/status/1378679960408371207


boot/vmlinuz-5.10.0-5-amd64 and boot/initrd.img-5.10.0-5-amd64-dracut are clean, from Debian

boot/bzImage has a Long Sleep in swap_writer_finish(), boot/initrd.img-5.10.0-5-amd64 logs each txg it commits


Boot the latter with 'verbose debug no_console_suspend boot=zfs', with a ZFS-on-root pool attached, make big enough zvol, mkswap and swapon it, run systemctl hibernate or equivalent, let 'er rip
You should get "S0|0!" instead of the usual "S|", then "bdm"/"bdm done" during the waits. Sometimes an NMI happens thereduring and a backtrace is printed;
you should get at least one (but likely a few) "4txg=..."s after "S0|0!" -- that's good

Now boot with 'verbose debug no_console_suspend', you should get dropped down to an i-t shell prompt
Now:
  modprobe zfs
  zpool import -aN -o readonly=on
  zfs load-key -a
should get you the device node and a read-only pool.
If at any point you write to the pool (like by importing it R/W), i.e. you see "4txg=..."s, stop trying to resume and boot normally, or you risk uncorrectable pool errors.

Great, if you now
  resume /dev/zvol/pool/swap
you should get a big load of shit that starts with "can't write to read-only device" and ends with "errno -5" or summat. This is good.

If, instead, you get "no image found", this means there's no image header, 'hexdump -C /dev/zvol/pool/swap | head' should have S1SUSPEND in it; stop, boot normally.

To actually resume this, you can
  modprobe zram
  zfs get -Hpo value disksize pool/swap > /sys/block/zram0/disksize
  dd if=/dev/zvol/pool/swap of=/dev/zram0
  resume /dev/zram0

No need to export the pool since it's unchanged by a read-only import and every live page is being replaced anyway.


These arguments might or might not make it faster/slower to hibernate, but aren't by any means needed
  cd /sys/module/zfs/parameters/
  echo 0 > zfs_arc_max
  echo 1 > zvol_request_sync
  
  echo 1 > zfs_txg_timeout
  
  echo 230:0 > /sys/power/resume
  echo 0 > /sys/power/image_size