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- Building a NAS - need hardware advice
December 9, 2005, 7:24 am
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basically a box with a whole bunch of hard drives for Terabytes of
storage goodness. I've already chosen the case (Cooler Master's CM
Stacker) that will allow me to add up to 12 HDDs. I'm having trouble
with picking the innards though as I don't know what the requirements
of a NAS machine are. BTW, in the future I might use this box for a
TiVo-like functionality (recording shows and such) - this might have an
effect on the final components.
The motherboard will be pretty vanilla, with integrated everything (I
just need video and network, no need for audio), plus USB booting for
Damn Small Linux (want to keep OS separate from actual storage).
- What kind of power supply do I need? Is 400W enough or should I go
for 500W or even 600W? Why?
- Do I need a fast CPU?
- How much RAM do I really need?
- What is the usual way to add additional IDE drives beyond the four
allowed by the motherboard? I think I've seen PCI controller cards -
how many drives can I add with each card? Can I fill up all the PCI
slots with these cards or is there a limit?
- Anything I've missed?
Thanks a bunch!
Re: Building a NAS - need hardware advice
A server doesn't need a lot of resources, for your home
configuration. In fact, using a lower power processor,
may make your power supply selection easier.
If you go to a disk manufacturer's web site, they will give
numbers for idle power consumption and read/write consumption.
In a home environment, your server might not be that busy,
in which case using idle power estimates for all the drives
might be good enough. In a corporate environment, you might
use the read/write current requirement, for your estimate, on the
assumption that all arrays are fully busy.
Say the idle power is 12V @ 0.5A and 5V @ 1A, then with 12 drives,
you need 12V @ 6A and 5V @ 12A of power more than a normal computer.
With extremely large numbers of drives, you should also consider
whether it is possible to implement "staggered spin". What that
means, is the disk drives start spinning sequentially. The
disk drive motor draws 2-3 amps from +12V, while it is accelerating
the spindle to its operating speed. Staggered spin is easier
to implement with some technologies (SCSI) than with others.
You can see, that the running current estimate above,
was 12V@6A, and if all the drives try to spin at the same
time, that initial surge of current is 12V@24A or more. So,
if you follow through on using a large number of drives, do
more research on staggered spin.
This document describes a scheme used by Maxtor. Basically,
you need two ingredients. A disk controller card that is
aware of the possibility that drives are set for staggered
spin, and the drive itself. If implemented successfully, the
power drawn when the computer first starts, might be 12V @ 10A
versus the 12V @ 6A once all drives are spun up. The 12V @ 10A
is certainly easier to meet, than the 12V @ 24A of a non-staggered
spin system - but of course you can always "brute-force" the
system construction, and use a power supply with a large
12V output (like one of the older Antec EPS550 supplies or a
PCP&C or Zippy/Emacs supply).
Here is a 12V@34 amps brute force solution. (ATX 2.0+ supplies,
that split the 12V rails, are not the same. You want a supply with
a single 12V rail, to get as many amps as possible to spin up
12 disk drives at once. An original Antec EPS550 with the
12V@36A output, would be another solution, but the TPII ones
that split the 12V rail, only give 19A to the drives.)
You can work out the base load of the computer here. The
answer will be in amps for +3.3, +5, and +12V, and many
internet retailers give the separate current ratings on the
power supplies they sell. Add your dozen disk drives to the
result (and add enough current for whatever spinup scheme
you can manage):
Using a brute force powering solution, and ignoring the
staggered spin approach, will make it easy to slap together
a big server. There are many high end supplies, with multiple
12V outputs, that could also solve your problem, but you
would want to rewire the outputs of the supply, to get the
maximum advantage of the way the +12V outputs are distributed
in the wiring harnesses. (A supply with four 12V outputs,
may put two 12V outputs for dual processors, one for the
motherboard, and one for the disk drive molex power connectors.
Rewiring one of the processor power outputs, to power disk
drives, would be a better use for one of those processor
Using two power supplies is another possibility, but you must
have a good understanding of powering issues (what happens if
one supply does not start when it is supposed to, and how to
hook two PSUs together, for safe operation). Buying a single,
properly engineered supply, eliminates all of that fooling around,
and the a PCP&C supply or a Zippy, is one way to do that. If
you use a lower end processor, that will keep the 12V consumption
low enough, so that the PCP&C will get the job done.
For more inspiration, try forums.2cpu.com:
For sheer excess, check this out :-)
Re: Building a NAS - need hardware advice
You're going to need a RAID controller. 3ware makes some decent 12
port SATA controllers. One piece of advice... WD SATA Hard Disks are
garbage. Out of about 200 I have I've already had to replace about
50 in the last year. I've been phasing them out in favor of Hitachi
disks and have not had any of those fail yet. Make sure you get a
motherboard with at least 1 64-bit PCI-X or PCI Express slot for your
RAID controller. Basically you will keep your OS on a disk attached
to the integrated IDE controller (You can boot off USB but it might be
more trouble than it's worth). All of your other disks will be
attached to your RAID controller and create an array (a RAID 5 would
probably work best in your case). This array will appear to the OS as
one huge disk that you can use however you like. As long as its
formatted with a standard file system (i.e. ext3, Reiser, XFS, etc.. )
you should be able to mount it under any standard linux distro. You
might also want to look into using LVM if you decide to use more than
one RAID controller card but I'm not going to go into that now. For
starting out I would recommend just using one 12 port controller since
managing multiple volume groups can get a bit complicated.
Hope that helps a bit.
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