The /sys/power/disk file controls the method to hibernate to disk.
Reading the files returns the available options:
[root@archlinux ~]# cat /sys/power/state
freeze mem disk
[root@archlinux ~]# cat /sys/power/disk
[platform] shutdown reboot suspend
[root@archlinux ~]#
If I want to suspend to memory I need to do:
[root@archlinux ~]# echo mem > /sys/power/state
Suspend to disk:
[root@archlinux ~]# echo disk > /sys/power/state
The suspend-to-disk method may be chosen by writing to /sys/power/disk one of the accepted strings:
'firmware'
'platform'
'shutdown'
'reboot'
'testproc'
'test'