IMac – Is hibernation possible on a T2 iMac / Catalina

imacsleep-wake

From time to time, I want to unplug my iMac from power in order to move it, but I also want it to restore the state back to where it was. I learned that I could enable a hibernation mode, which would save the contents of RAM to SSD before sleep:

sudo pmset -a hibernatemode 25

Now, when I tell it to sleep, I can hear the fans turning off after 10 seconds. But when I wake it up, the screen turns on instantly, as if no sleep was happening. This is clearly wrong, because the documentation says the memory should be powered off:

hibernatemode = 25 is only settable via pmset. The system will store a
copy of memory to persistent storage (the disk), and will remove power to
memory. The system will restore from disk image. If you want "hiberna-
tion" - slower sleeps, slower wakes, and better battery life, you should
use this setting.

Just to confirm, I powered off/on my iMac and it booted up from scratch without restoring any memory.

Looking at pmset -g, everything looks fine:

System-wide power settings:
Currently in use:
 autorestart          0
 Sleep On Power Button 1
 halfdim              1
 hibernatefile        /var/vm/sleepimage
 proximitywake        1
 powernap             1
 gpuswitch            2
 networkoversleep     0
 disksleep            10
 sleep                10
 hibernatemode        25
 ttyskeepawake        1
 displaysleep         10
 tcpkeepalive         1
 womp                 1

I also checked that /var/vm/sleepimage does exist, occupying ~17 GB (total RAM is 32 GB).

Has anyone been able to enable hibernation on a desktop Mac recently, or is it just some issue with my machine?

I'm running an iMac Pro (2017) with T2 chip on Mac OS Catalina (10.15.7)

Best Answer

I suspect that some power is still required to maintain hibernation, as distinct from 'off'. So you may be able to hibernate an iMac, but only while it's still plugged in.

The 'Resume' feature of macOS is designed so that desktops can relaunch apps and open documents and windows that were up at shutdown, 'resuming' where the user left off.

sleepimage does exist on my 2018 Mini, over 2GB in size: though the Creation and Modification date are both 8 November 2018, which was probably when I first turned it on.

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