[mythtv-users] Backend hardware advice - RAID suggestions - RAID TCO Comp Calc Spreadsheet

Brad Templeton brad+myth at templetons.com
Mon Jul 17 01:14:45 UTC 2006


On Sun, Jul 16, 2006 at 02:53:26PM -0700, dave johnson wrote:
> I hate to have to put this so bluntly, but you are blatantly and absolutely wrong Brad.

Well, what you point out is that my logic doesn't apply 6 months ago.
But that's not relevant as advice for people building a system today,
which is I presume what is worth talking about here and today.

So that's not blatantly and absolutely wrong.  That's subtly wrong in
a way that isn't still relevant.   There is a difference.
Neither person is blatantly and absolutely wrong here, it's a subtle
matter -- your first instinct to not put it bluntly was the right instinct.
I'm not trying to insult you, just saying that it's worth factoring
in electricity into the economic (and environmental) equation of
any large always-on server, and you didn't address that.  So chill.
(literally and figuratively.)

You got a good rebate price on some drives, and by bending the rebate
rules and spending the time that takes.   Drive prices vary (and
decline with time) and rebates come and go, but the reality for
people building systems is that there's usually a sweet spot with
the lowest price per gigabyte (looks to be between 300 to 500gb
right now) but the price/gig doesn't vary a great deal over most of
the middle range.  (It usually gets high in the largest drives such
as the 750gb which are costing over 50 cents/gig) and the very
smallest.

However, my point, which is still quite valid, is that in the
middle range, the normal modest savings at the sweet spot do not
compensate for the extra power cost of buying fewer drives above
the sweet spot.

Each 24/7 drive tends to cost perhaps 120 kwh/year, plus cooling cost.
Or about $20/year in power, $60 over a 3 year lifespan.  I am not
factoring in costs of extra controllers.  AC costs are usually rated
as 1/3 extra during AC season, heating offset I don't know, it may
balance that.  (This is at bay area incremental power cost.)

So if you have 3 drives costing less than $60 over 4 drives, the 3
drives are the eway to go, though the RAID-5 equation changes that a bit.

I wasn't addressing the issues on hardware raid vs. software raid at
all, other than the fact hardware raid may add slightly to the power
cost.   Having so many drives to need 2 power supplies also adds to
the power cost.

Of course, if you are going to build an expandable array of drives,
the right strategy, especially if you live near Fry's, is to buy
only enough disk space to handle what you need, and then add more
drives as the need arises.  Then you aren't paying for drives and
power for them until you need them.   Perhaps you filled your 2TB
immediately, but if one does not, it's always wiser to wait.


> Here you go people : 
> http://se30.com/users/grindingbassline/mythtv/RAID_TCO_Comp_Calc_v200607161445.zip

I've seen numbers like 12w, and it varies from drive to drive, but I
have not actually measured it.   You want to measure not the current
to the drive, but the total watts into the power supply with and without
the drive because the power supply has inefficiencies (about 10%) and
there are other factors in system.

Your chart places 6 extra drives (11x200 vs. 5x500) costing just under
$85 per year in power.  Add some for extra controllers and power supply
and I would think you would agree it's near to $100.

Leaving out below-cost rebates, which are fun and worth it if you
have the spare time, looking at Fry'I see $57.77 for 200gb 
and $119 for 400gb, which are 30 cents/gig.  The 500gb special
was last week, I guess, at $189 for 500gb (38 cents).
Obviously the 400gb are better than the 200gb.  5x500 ($945)
compared to 11x200 + extra controller plus extra ps ($660 plus what?
I will guess it goes to $800) shows a $150 difference.

And that's paid for in 1-2 years.


I appreciate you spent a lot of time and used tricks to get
rebates on lots of drives.  I hope you understandy why that's
not germane to the debate on the best way to design a system.


This site is handy for tracking the price of drives:

    http://www.frys-electronics-ads.com/


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