RAID

The one of the latest storage technologies is RAID6, an important new technology for image management system data storage. You can read a white paper on RAID6 here, and another (rocket scientists only) here (reg. req.). Along with all this new RAID tech is word that you could lose all your data by using RAID5 with SATA drives. That's right, ALL YOUR DATA. After you read the blog post, be sure to scroll down for some great comments. Since I don't know how long InfoWorld will keep this post online, I'll excerpt a few comments below:

I remember reading almost exactly the same white paper when 9.1G
SCSI drives first came out - close to ten years ago. 9.1G! On one disk!
Amazing! You lose two of those and you are *screwed*. Well, guess what
- the new SATA drives have similar MTBF as the old 4.3/9.1 SCSI's did,
and those seemed to work pretty well - many of them are still in use.
(original 9.1G Seagate Hawk 800,000MTBF, 5 year life 5 year warranty,
new Seagate Sata NL35XX for SAN's 1,000,000 MTBF 7 year life, 5 year
warranty)

Raid5 with an automatic hot standby pretty much solves the issues
raised in the white paper (ie human error, slow response to drive
replacement, etc). And unlike Raid6, Radi5+hot standby is supported by
pretty much every new server out there...

dateline 2016: Don't use the new 4TB drives in Raid5...see white paper for why...

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MTBF isn't what hurts you with large drives; you have to worry about non-recoverable read errors.

Fibre channel drives typically have a non-recoverable read error
rate of 1 sector per 10^15. SATA drives typically have a
non-recoverable read error rate of 1 sector per 10^14. So you are 10
times more likely to have a non-recoverable read error on a SATA drive
compared to a FC drive. That assumes the drives are the same size!

During a RAID5 reconstruct, the system may read every sector on all
the surviving disks. If anyone of the surviving disks has a
non-recoverable read error the reconstruct will fail. It's the same as
having a double disk failure.

If you do the math, during a RAID5 reconstruct, a 144GB FC disk has
a 1 in ~3.5 million chance of a non-recoverable read error. A 500GB
SATA drive has a 1 in ~ 100,000 chance.

That is where the problem lies.

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Not technical enough for you? That's what David Schwaderer had to say (sent by e-mail)

Oh please, I find the Infortrend RAID 6 white paper pathetic. It
does not even touch on what the so-called "RAID 6 multiplication" is
and how to perform the matrix inversion calculations for recovering
double data loss.

A real one I wrote for Veritas that completely explains all RAID 6 foundations is posted on the Veritas Architect Network here.

Thanks for all considerations,
David Schwaderer

The past is not dead. In fact, it's not even past. - William Faulkner
The future is already here. It's just not evenly distributed - William Gibson, "Neuromancer" Author

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I can't help but notice that the RAID6 "standard" is presented by
Infortrend, the very company raising the alarm about RAID 5's
"vulnerabilities." Like most WPs I see, this one is a self-serving
advertisement masquerading as objective content.

Not to say that RAID6 isn't cool and useful, I'd just rather hear it
from a truly objective source, like an independent university research
lab.

Aren't blogs great?