suppose power is scarce, like in airplane, and what the laptop is being used for is not at all taxing on the machine’s resources, like reading a book or writing texts. In this situation I think that one of the ways to save power would be to shut down hard drive and have the machine run purely on RAM. So the idea is that we turn on the machine, load all the needed data from disk to RAM, turn off the disk and work using RAM. If there is anything to save along the way (a big if, especially if I am just reading) I save it to flash.
Can Windows OS be told to do so? How about the various *nix variants intended for the desktop/consumer?
Most modern OS’s will spin down the disk after a certain period of inactivity (and, as once was the case with Ubuntu linux, sometimes too aggressively). Some of the system will be cached in memory, but a modern OS is a big beast, and systems generally don’t have enough memory to cache everything. However, many linux live CDs cache a significant proportion of the OS in memory, and a specific purpose distribution like Damn Small Linux or Puppy Linux will run entirely out of memory after boot. Moblin and the OLPC also use similar techniques.
Si
Windows Vista and 7 have a technology called ReadyBoost that will use a USB memory device as an alternative to disk for caching. This gives some of the benefits of an SSD, although the algorithms used in Vista were reportedly not great; Win7 is said to be better at it. I have a 4 GB key on my work desktop used for ReadyBoost, and I do notice that the hard drive is accessed a lot less frequently, although the USB key flashes a fair bit (it has it’s own “activity” indicator). I haven’t done experiments to see if I can get the hard drive activity to zero, nor do I think Windows provides a way of shutting down the hard drive while the system is still in use.
For Windows, at least, shutting down the hard drive is a function of inactivity of the user, not access to the hard drive.
If you look through this PDF, particularly pages 8-10, you’ll note that the amount of power being used by the hard drive is negligible except when it’s actually in use. When the system is idling–like it would if you were reading–the HDD is only eating about 4% of your power, so even if you cut it out entirely, you’ll only get a 4% saving. Changing the monitor to display less brightly gains you a 29%+ saving.
More importantly, all other components are similar to the HDD. When they’re in heavy use, they’re eating most of the power, when they’re not, they’re hardly taking anything. The system is designed to be that way. The only thing which doesn’t modify itself based on usage is the display. If you can live with that being less bright, it’s up to you to dim it.