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To Sleep, Perchance to Dream

The MacBook and MacBook Pro feature something called Safe Sleep. Put simply, when the computer sleeps, Safe Sleep performs a backup of everything in memory and places it on the hard drive. While in progress, you’ll notice the sleep status indicator light on the front of the computer glows steadily; this is a warning not to move the computer because the hard drive is still spinning. When it’s finished writing to the hard drive, this light will start pulsing, signaling that it is now safe to move the computer. This feature exists primarily to allow you to change batteries without losing data. Because they don’t have built-in backup batteries, (unlike some earlier Mac portables) without Safe Sleep, removing the battery from a sleeping MacBook or MacBook Pro would cause them to shut down, unless you had them plugged into an outlet. But with Safe Sleep, if you happen to remove the battery from a sleeping computer and lose the RAM contents, your Mac restores that data from the copy made on the hard drive, and you can resume work.

This Safe Sleep mode is a useful feature, but not everyone it is a fan. Safe Sleep requires a great deal of disk space. If you have 1.5 GB of RAM installed on your notebook, then Safe Sleep will commandeer 1.5 GB of hard drive space for its purposes. You can see this file (called "sleepimage") by using the Go To Folder command under the Go menu in the Finder, and typing /var/vm and then hitting the Go button. Because this file is only used if the RAM contents are lost, it isn’t very useful if you keep your notebook plugged into an outlet or only have one battery that doesn’t get completely discharged. Under those conditions, your MacBook or MacBook Pro wakes up using the contents of RAM rather than the backup it made on the hard drive.

Safe Sleep is also slow compared with the plain sleep mode of older Mac portables. A PowerBook G4 will go to sleep almost instantly, and as soon as you put the computer to sleep, the hard drive will stop spinning and you can safely move the computer without risk of damage. Doing the same thing with a MacBook Pro takes longer, and it will take 15-20 seconds for all the data to be copied to the hard drive and for the sleep status indicator light to switch from the "warning" glow to "safe" pulsing.

Safe Sleep also doesn’t actually save any energy, because while the information in memory is copied to the hard drive just in case the RAM chips lose power, it doesn’t deliberate power down the RAM chips in the same way as Hibernate mode does on many notebook PCs.

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