[Ukfreebsd] "green" HDDs

[Tim Borgeaud]

Frank Shute wrote:
> 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.
> 

Indeed. A disk that is spun down frequently is much more likely to show 
some large disadvantages performance wise, in most applications, compared 
to a disk run at full speed all the time. However, I was under the 
impression that the power savings features of these green disks don't 
geneally involve the disks spinning down and sleeping automatically but 
are centered around two features:

1) Automatic parking of the heads after a short (too short in my opinion) 
time. This, apparently causes slightly less drag on the platters and saves 
a little bit of power while the disk idles. It may also be accompanied by 
some non-mechanical power savings changes.

The cost is the extra time it takes the heads to swing back over the where 
the data is located. However, perhaps the larger problem is where there 
are relatively periodic access separated by more than the timeout, causing 
a continuous cycle of head parking/unparking.

2) A possible automatic lowering of the spin speed while idle, such that 
the disk does have to speed up again for access but can do so relatively 
quickly.


If the disks are spun down, either due to disk firmware or due to a 
standard OS utility (such as ataidle), I suspect that the timeout will be 
set to a much larger value than a few seconds.

Tim