Drives use by far more instantaneous current to spin up then they need to
run in active idle mode. A big, modern IDE drive can consume about 2.0A
to spin up, but probably no more than 0.5A to sit in active idle. (Note
that the "." is a decimal point; I'm in the USA.)
For example, I looked up the Western Digital WD307AA, which is what we are
currently using two of in our RAID-1 set. This drive is rated to require
1.8A on the 12V line and 0.530A on the 5V line to spin up, but only 0.240A
on 12V and 0.660A on 5V when reading, writing, or idling.
Then I checked on the capacity of a reputable power supply vendor, PC
Power & Cooling (www.pcpowercooling.com), and looked up their bottom of
the line power supply, the "Ultra-Quiet Silencer 235 ATX." It is rated to
supply 22A at 5V (subject to a total dissipation rating on the 5V and 3.3V
lines combined) and 8A on the 12V line. So, spinning up four WD307AA
drives simultaneously -- a total of 120 GB of raw storage -- would still
be more than 67% below the rated performance of the system on the 12V
line, which is used by little except hard drive motors and positioners.
Of course, if you decided not to use the bottom of the line supply and
instead bought, say, a "Turbo-Cool 300 ATX," then you would be allowed 30A
on the 5V and 12A on the 12V lines, an enormous extra margin of safety.
As you can see, this also suggests that spinning down a drive is a bad
strategy for a RAID set if all of the drives are likely to be spun up
simultaneously. Spinning down drives may be a good idea for other
reasons, such as battery conservation in a laptop, but it would probably
lead to more frequent peak demand being placed on the power supply, with
an attendant risk that the drive might not spin up when called for.
To be honest, the 250W power supply is a grossly overdesigned relic of a
different era when drives sucked down real power. We used to have a
standard configuration for NetWare/286 servers that used full-height
Seagate ST4383E (ESDI) drives in a mirrored configuration. I looked up
the specs just now, and I see that each of these required 4.5A at 12V and
1.4A at 5V to spin up, and required 2.0A at 12V and 0.8A at 5V just to run
normally. We had no trouble with two of these on 250W supplies, so I
can't imagine that any modern drives would be a problem.
-- Mike
On Fri, 7 Apr 2000, Jakob Østergaard wrote:
> On Fri, 07 Apr 2000, Sven Kirmess wrote:
>
> > Thursday, April 06, 2000, 10:08:59 PM, Jakob wrote:
* * *
> > > But I fail to see how this relates to the size of the PSU ? All
> > > disks must run when you use them, and if you need a 300W PSU to do
> > > that, you can't use a 150W even if your disks are only in use 50% of
> > > the day. (Obviously)
> >
> > I know that of course. My question was more like "Is there a power
> > problem with a couple of IDE disks?" or "are you running special
> > supplys?".
>
> We have six 6G old Quantum SCSI disks in a home-made RAID tower here.
> The disks spin up all at the same time (they're to stupid/old to do
> anything else), and they run of a cheap AT PSU. No problems, but
> I have no idea what the load is on the PSU when they spin up.
>
> > I won't create a RAID system and loose more data because of a too weak
> > supply than I would loose during a disk failure.
>
> I think that if you get past a synchronous spinup, you'll survive
> anything that might happen during use. But this is all a ``may''
> and ``might'' discussion...
