Here’s a tip for those of you who, like me, back up your data to hard disks instead of tapes. Backing up to the same hard disk doesn’t protect you much (if the disk failed, you’d lose the data and the backup at once), so presumably you’re backing up to a separate physical drive. That means that the backup drive need not spin 24/7. Instead, it only needs to spin at backup time.
Fortunately most disks can be told to spin down when idle, like laptop drives do. For the main disks of a server this is probably not worth the trouble, but for backup drives it can save you a lot of power and heat. Excessive heat kills hard drives, so this can also prolong the life of your backup drive.
On Linux this is accomplished with hdparm:
sudo hdparm -S 6 /dev/sda
The numeric value is on a nonlinear scale, so man hdparm and read about the -S option to make sure you pick the value you intended.
You can check the current status (active or standby) like this:
sudo hdparm -C /dev/sda
External drives aren’t managed directly by the kernel, so hdparm won’t work on them. My hardware RAID card won’t pass through these commands. Linux RAID will not pass these commands through, though you may be able to apply the commands individually to RAID set members (which is what I was able to do).
I don’t know how to do this on a per-drive basis on Mac OS X ( pmset apparently affects every drive on the system), or at all on Windows. If you know feel free to post a comment explaining how it’s done.