-----Original Message-----
From: Justin Piszcz [mailto:jpiszcz@lucidpixels.com]
Sent: Monday, December 22, 2008 10:25 AM
To: David Lethe
Cc: linux-raid@vger.kernel.org; smartmontools-
support@lists.sourceforge.net
Subject: RE: Spread Spectrum (SSC) Enable or Disable?



On Mon, 22 Dec 2008, David Lethe wrote:

quoted

quoted

-----Original Message-----
From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
owner@vger.kernel.org] On Behalf Of Justin Piszcz
Sent: Monday, December 22, 2008 7:59 AM
To: linux-raid@vger.kernel.org
Cc: smartmontools-support@lists.sourceforge.net
Subject: Spread Spectrum (SSC) Enable or Disable?

Besides the power supply/surge/etc with a lot of drives-- I have

read:

quoted

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http://www.wrightthisway.com/Articles/cat_reviews.html
"As I mentioned earlier, the enclosure will definitely handle

drives

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using Spread Spectrum Clocking (SSC), an increasingly common

feature

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that helps reduce electromagnetic interference, especially between
drives in close proximity with each others, such as you might have

in

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a

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RAID setup, so that is a definite plus here."

With 16-24 disks-- if they are close together, it sounds like a

good

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idea to use Spread Spectrum Clocking?
Does anyone here with a large-ish raid array use this?

Any enterprise-insiders care to comment?

Justin.
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I know for a fact that Xyratex supports SSC on SOME of their

enclosures.

quoted

However,
there is a caveat to be aware of ... some RAID
controllers/firmware/enclosure combinations
have problems "seeing" disks that have SSC enabled.  Don't just turn
this on unless you
verify with the enclosure manufacturer that they support SSC for

your

quoted

desired combination.

David

Dave, thanks for the reply, in my case (and most people on this list)
use
their arrays in a regular ATX/case.

The question I had was:

"especially between drives in close proximity with each others, such

as

you might have in a RAID setup, so that is a definite plus here"

Besides the staggered spin-up functionality (which is enabled, at
least with WD drives when you enable SSC, which is good for the PSU)

is

there any other 'benefit' to using SSC in relation to the comment of
drives being close to one another?

e.g., have studies been done looking at several raid arrays with ssc
enabled vs. disabled and what / if any correlation can be made within
respect to failure rates?

Justin.

I can't imagine it having any positive effect.

SSC has to do with reducing overall electromagnetic emissions (EMI) for
FCC cert by 
spreading around clock harmonics a few MHz. 

All the drive vendors run reliability testing in the more expensive,
heavy-gauge metal
enclosures for a reason.  Torque/Vibration kills data, so they know they
need rigidity.
I view SSC as a cheat code that enables them to pass the FCC tests by
removing shielding
which will save them some money, and make a less reliable, more
error-prone enclosure.

I avoid any enclosure/RAID vendor that advertizes SSC as a requirement
(compliance/support
is different). This lets them get away with selling something a little
more flimsy, 
therefore, less reliable, then  what they would normally have to sell to
meet FCC certs.  

If I was in market for low-cost enclosures, I would see if they ran
their FCC compliance
testing with SSC enabled.  If they did, don't buy their enclosure.

David