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man says

   pm-suspend
       During suspend most devices are shutdown, and system state is saved in RAM. The system still requires power in this
       state. Most modern systems require 3 to 5 seconds to enter and leave suspend, and most laptops can stay in suspend mode
       for 1 to 3 days before exhausting their battery.

   pm-hibernate
       During hibernate the system is fully powered off, and system state is saved to disk. The system does not require power,
       and can stay in hibernate mode indefinitely. Most modern systems require 15 to 45 seconds to enter and leave hibernate,
       and entering and leaving hibernate takes longer when you have more memory.

   pm-suspend-hybrid
       Hybrid-suspend is the process where the system does everything it needs to hibernate, but suspends instead of shutting
       down. This means that your computer can wake up quicker than for normal hibernation if you do not run out of power, and
       you can resume even if you run out of power. s2both(8) is an hybrid-suspend implementation.

From the description, I still don't understand how the last one is related to and different from the first two. E.g., it can work when running out of power, and then isn't it also saving to the disk, and thus same as the second?

Thanks.

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Manual says it all, pm-suspend-hybrid basically do both: suspend and hibernate, i.e. saves system state to RAM and to disk. pm-suspend-hybrid is the same as pm-hibernate only if your system will experience power cut, but if power is present all the time when suspended then pm-suspend-hybrid is more like pm-suspend because system will wake up as fast as in pure pm-suspend.

In fact you can forget about first two possibilities and use pm-suspend-hybrid all the time. The only drawback is that saving state to both RAM and disk is obviously a little bit slower.

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  • Thanks. "pm-suspend-hybrid is the same as pm-hibernate only if your system will experience power cut". But how does pm-suspend-hybrid foresee whether and when I am going to cut the power or the battery is run out? If that happens, isn't it too late to write to disk from RAM? – Tim Oct 18 '14 at 19:25
  • @Tim I mean power cut when the system is suspended. After pm-suspend you will lose unsaved data and need to reboot, but after pm-hibernate or pm-suspend-hybrid your data are safe. – jimmij Oct 18 '14 at 20:59
  • @Tim The approach seems obvious: pm-suspend-hybrid first saves the RAM on the disk as pm-hibernate would do, but does not switch off the power. Then, it just performs a regular pm-suspend. When you press power button (or open the lid of your laptop), it just resumes from a regular pm-suspend. If power is cut while computer is in "suspend" mode, then all data in RAM is lost. Computer starts from beginning. However, since there is already pm-hibernate data on the disk, the computer resumes as if it is simply "hibernated" without knowing the "suspension" thing. This is what I guess. – ceremcem Oct 4 '20 at 11:49

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