Automatic means for spinning down disks available?

Chuck Swiger cswiger at
Wed Apr 11 23:14:35 UTC 2007

On Apr 11, 2007, at 3:32 PM, Gary Kline wrote:
> 	Some things to consider (besides powering -down or -off drives)
> 	are battery backup system.  Don't most UPS systems isolate your
> 	servers from the wall-socket?

The better grade of UPSes do exactly that-- they provide "galvanic  
isolation" by using an isolation transformer which has the primary  
and secondary windings completely separated, and ensuring in the  
design that you don't connect the service neutral line to the output  
or load's neutral line.  The load can thus either be floating or tied  
to the local building ground.  This type of design is known as  
"double-conversion" because they always feed the input AC line  
through the rectifier & DC inverter, using more power but providing  
better PFC and can provide the load with an AC frequency which is  
different than the input AC frequency (ie, they can provide 60Hz  
output from 50Hz input, or vice versa).

Cheaper UPSes, which include almost all consumer-grade models from  
APC, Tripplite, etc run in "line interactive mode", which involves a  
self-tapping or ferro-resonant transformer, can adjust the voltage up  
or down within limits, but they do not perform PFC and cannot provide  
frequency conversion, and they pass the neutral line from AC line to  
load without isolation, thus passing common-mode noise through.  This  
design is lighter and requires fewer components (an isolation  
transformer is heavier), and does not keep the DC section and  
inverter always under full load, so are somewhat more efficient, but  
cannot deal with frequency drift or significant voltage changes.

>   At what level do hard drives have identical circuitry so that  
> they can be software lower-voltaged?

The boards within a drive family might be identical (WD200BB/WD400BB/ 
WD800BB/etc), but they don't deal with under-voltages at all well--  
you'll either pull excessive current through the servo and spindle  
motor windings, or perhaps the drive will fail to spin up entirely.   
The spindle motors are designed to spin at the calibrated speed and  
won't spin at slower speeds.

> 	*Except for consumer __cost__*, why don't all boxes have builtin
> 	batteries like latop?  ...There are lots of things to consider.

Cost is the primary reason why boxes don't have built-in batteries.   
People flinch away from paying for real RAID systems which include  
battery-backup for the drives...


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