Comment 132 for bug 59695

@Blue:

Regarding this: "Even more, on the same manufacturer's site I found a document where they say that respinning up a harddisk takes a lot of power (the current peaks at about 1A) which means that if it's needed/done too frequently it basically nulls any power economy you would make by spinning the drive down in the first place..."

I don't think this picture is entirely accurate. Typical drives draw ~1 W while spinning without reading/writing, ~2 W while reading/writing, and ~2-4 W while spinning up. The break-even point depends on the exact values and the spin-up speed, although I guess that the total amount of power required to spin up will be somewhat constant (i.e., a longer spin-up will require a lower wattage). I did some measurements on this a while ago, published here:

http://www.linuxjournal.com/article/7539

You can see from the chart that spinning this particular drive down *every* 12 seconds already yields significant savings. This suggests that the break-even point lies somewhere around 6-8 seconds of spun down time.

Now, regarding the "insane" -S4 setting for laptop mode: this setting is intended for battery mode only, and only on laptop drives. Let's do the math. Assuming you spin down the drive *only* while you're on battery (which is when it matters), and you get one spindown every minute. Assume the laptop lasts for 5 hours on every charge (a high estimate for typical laptops) and the battery has a lifetime of 1200 discharge-charge cycles (again, a high estimate). Then you have 1200 * 5 * 60 = 360000 cycles before you have to replace your first battery, and then you can take another 800 cycles before the 600000 spindown mark is reached.

Alternatively, consider when you use battery mode for exactly 5 hours a day, every day (a quite extreme situation). That's 300 spindowns per day, or 300*365 = 109500 spindowns per year. That yields a lifetime of 5.48 years (again assuming a lifetime rating of 600000 spindowns).

Note that one spindown per minute is a very high average, you'll almost never hit that: usually, you have more extended periods of continuous drive usage (when you're doing stuff), as well as more extended periods of no drive usage (while you're reading stuff or editing a document/file). Also, five hours of on-battery usage on average *every day of the week* for years in a row is a very high estimate (I guess most laptops get used fully for either 5 or 2 days a week, not seven). I therefore think that -S4 is a pretty safe setting for on-battery usage. Also, setting it higher will cut into your power savings very fast. Set it to -S12 and your drive will probably not spin down very often at all, which means that you can just as well turn laptop mode off.

Anyway, I think that any power management settings which make a drive load/unload once every minute *all the time* are doomed to kill drives. No need to blame this problem on the -S4, which is for a very special use case (on battery) only.