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...

--
-Chuck

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