Discover Safe Sleep's secrets

While reading this thread on the Macworld forums, it struck me that very few people are really aware of Apple’s Safe Sleep feature, and exactly how it works on newer laptops.

Since roughly fall 2005, all Apple laptops have supported Safe Sleep mode. Some older machines can also support Safe Sleep—you’ll have to be willing to get your hands dirty in Terminal, but it does work. Just follow the instructions in this hint —I know it works for at least some older Macs, as I used those steps to modify my 12-inch PowerBook G4, which predates the fall 2005 timeline.

So exactly what is Safe Sleep? Basically, the contents of your machine’s memory (RAM) are written to disk before the machine goes to sleep. If it then loses all power while sleeping, it will “wake up” the next time you power it up by reading back the saved RAM state. This all happens automatically—the only sign you have that it’s happening is that there’s now a delay before your machine’s power light goes from solid-on to blinking-on. During this time, you should not open your Mac up again—technically, nothing bad is supposed to happen, but I’ve read reports of people finding unresponsive machines if they awakened them during this stage of the sleep process.

Once the machine is asleep, you can treat it as you usually do. But if you forget about it (you get caught up in all seven games of the World Series, for instance), you can rest assured that you can pick back up right where you left off. Even better, if you’re on a long cross country flight, you don’t have to shut down before swapping batteries. Just put the machine to sleep, wait for the glowing light, and then remove the battery. Insert the new one (no rush; your data is safely written on the hard drive, so it’s not going anywhere), open the lid, and wake up the sleeping machine by briefly touching the power button.

So this all sounds great in theory. How’s it work in practice? To demonstrate, I created a video showing exactly what happens, from start to finish. (If you’d like, you can download the original 13MB QuickTime movie version.) As you’ll see in the video, despite having a number of large, complex apps open, absolutely nothing happens when I yank all power to the machine after putting it to sleep.

Safe Sleep gives me one less thing to worry about when I leave my MacBook sleeping for an extended period—and it saves the annoying “quit everything and shut down” process when I want to swap the machine’s batteries in flight. Thanks, Apple, for including this most useful (if generally hidden) feature.

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