[Ukfreebsd] "green" HDDs

Frank Shute frank at shute.org.uk
Wed Jan 12 08:36:37 GMT 2011

On Tue, Jan 11, 2011 at 01:15:26PM +0000, Tim Borgeaud wrote:
> Despite the fact that green credentials of these drives may be 
> questionable, I think that in typical usage there are some definite 
> benefits.
> As far as I can tell, the performance of the somewhat slower spinning 
> (5400 rpm) drives is not really that terrible at all. I believe that the 
> slower spinning drives compare poorly in terms of random access, but 
> overall there isn't a huge difference (at least not between similarly 
> priced drives).

If the disk is parked, then I can imagine that reads or writes are
going to be slow; I'm assuming that it has to be run up to full speed
before it can be written too.

> I suspect that the very aggressive power management features won't make 
> any significant difference other than giving the drive a particular set of 
> statistics with which it can be compared to others. However, I also 
> suspect that in the vast majority of cases, drives will be idle for 
> virtually the entire time that the host computer is turned on. I, 
> therefore, think that any additional power and strain caused by waking 
> from any power saving state is likely to be negligible.

I think it depends on whether the machine it is in is tasked as a
server or a desktop. My webserver gets a hit every 5 mins or so, so if
it's not cached then it's going to be spun up.

> Where the drives may have some advantage is where is it useful to keep the 
> overall power of a system low. This may be for battery powered devices or 
> it may be to allow a compact or very quiet computer. It's probably also 
> worth noting that it is often the case that the slower spinning drives are 
> also slightly quieter than the faster drives.

I can see them being of some use in laptops, like you say.

I did once emulate one of these disks with my laptop and Linux. I
disabled fsync() from being called so often by periodic (I think) and
tuned the HDD with hdparm to spin down after x mins of inactivity. It
was about 10 yrs ago so the details maybe sketchy/wrong.

It worked OK but then I was only using an editor away from the mains.

Latency was horrible. I've no idea how long it extended battery life
by, as I never tested it.

> I also suspect that there may actually be some small benefit in power 
> savings when considering large quantities of such drives in situations 
> where the idle power of systems is relatively low (where a watt or two per 
> drive might actually make a difference).

As you say, a server farm or similar might be a possibility but you'd
have to work round the latency.




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