[mythtv-users] Software RAID

[Matthew McClement]

matthew.garman at gmail.com wrote:
> On Fri, Mar 09, 2007 at 11:35:22AM +0000, Matthew McClement wrote:
>> I've always sized my supplies based on the total *max* draw of all
>> the drives combined. This is not the figure on the drives
>> themselves, you usually have to dig around the spec sheets for
>> this figure.
> 
> And the max power draw almost always occurs when you power on the
> machine.  IIRC, startup power for a drive can be on the order of 30
> watts, where regular running is maybe 10.  So if you have 10 drives,
> and try to start them all at once, you'll have an instant 300 watt
> demand, but steady-state operation is only about 100 W.

I've seen my drives hit within 10% of max draw on a heavy RAID to RAID
transfer(measured using a watt meter on the wall and compensating 20%
for PSU conversion loss). It's rare, but quite frankly I'm not prepared
to risk undersizing a power supply, and oversizing doesn't lose you
anything except maybe a bit of money.

You also need to compensate somewhat for general wear on the PSU, the
rated value won't remain static. Especially in a home situation where
the environment won't be a nicely aircon'd 20C DC that's friendly to all
the heat producing bits.

> That is one nice thing about the expensive hardware RAID cards: they
> usually have an option to specify spin-up delay on a per-drive
> basis.  Theoretically you could power one drive at a time, and keep
> your PSU load reasonable.

Quite a few motherboards with a high number of SATA ports now support
staggered spinup, my BIOSless* Supermicro cards sort-of do it(if you
listen to the drives it spins up 4 drives at power on, the remaining 4
at disk detect) and I've seen some of the cheaper 4 port cards with a
BIOS support it as well.

However, with SATA this isn't a guaranteed feature. There are quite a
few drives out there(older Deskstars, low end Maxtors and some of the
consumer WD's are ones I know about) don't support staggered spinup as
they don't pay attention to ATA commands until the drive is fully powered.

Matt

*BIOS here being defined as something you can enter and change
parameters with. I know pretty much every SATA card must have some kind
of firmware/BIOS on it regardless of whether you can change anything or not.