[NLRS] Hard Drive Backup & Redundancy - Please be done

[S. Earl Jarosh]

John,

Thank you for the most elegant and comprehensive example and explanation of
RAID technology.

Can we please let this thread die now? 


S. Earl Jarosh, N0HZ
V.P. of Information Technology
Cell:  612.868.1313
Off:   763.545.3275
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earl at moneycenters.com
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-----Original Message-----
From: nlrs-bounces at mailman.qth.net [mailto:nlrs-bounces at mailman.qth.net] On
Behalf Of John P. Toscano
Sent: Tuesday, July 10, 2007 7:31 AM
To: NLRS
Subject: [NLRS] Hard Drive Backup & Redundancy (was: WA2VOI DISASTER)



Jason Godfrey wrote:

> OK, I used a bad choice of words. I don't consider RAID a "problem".
> (In fact, I've been trying to convince a relative to get some business 
> critical stuff on to RAID.) But I've encountered people that assume 
> that because they have RAID they are safe, which isn't true. I've seen 
> as many computer have to be installed from scratch due to software 
> issues as from lost drives. RAID won't help you with that, backups 
> will.
> 
> RAID is very useful tool. It is not the end all solution to that which 
> threatens your data (in which I include applications.)

RAID can be protective of your data, or can increase the risk of data loss,
depending on which "level" of RAID the disk array is configured for.

In brief, RAID 0 (striping) is a performance-oriented solution, but a
2-drive RAID 0 array is twice as likely to fail as a single drive, because
failure of either drive renders everything unusable and unrecoverable on all
the drives.  And a 4-drive RAID 0 array has 4 times the risk of failure, and
so on.  Just for the sake of simple numbers, if the failure risk of one
drive is 1:1000, the risk of a 2-drive RAID 0 array is 2:1000, etc.

RAID 1 (mirroring) is a reliability-oriented solution. Each primary drive
has an exact mirror image on a secondary drive. If either drive fails, the
other drive still has all the data, so no data are lost unless both drives
fail. If the risk of failure of any drive is 1:1000, then the risk of
failure of a RAID 1 array is (1:1000) x (1:1000) = 1:1,000,000.  The main
downside is that your net storage capacity is half the size of the sum of
the individual drive capacities, e.g. two 100 Gb drives add up to 100 Gb of
storage.

RAID 5 (striped set with distributed parity) requires a minimum of 3 drives,
but I would say that a 4-drive array is probably more common. 
This can yield most of the performance of RAID 0 with most of the
reliability of RAID 1. In a RAID 5 array, if one drive fails (or is pulled
out), the distributed parity allows the remaining drives to compute the
missing bits, and provide all of your data transparently. 
You then pop in a fresh working drive in place of the failed one, and the
array re-builds itself.  For a 4-drive array of drives with a failure risk
of 1:1000, the net chances of data loss would be (1:1000) x
(3:1000) or 3:1,000,000.  This is because the first drive failure
(1:1000) causes no data loss, but if any of the remaining 3 drives fails
(3:1000) before the array is re-built, then your data are gone. An array of
four 100 Gb drives has 300 Gb of user capacity, with the other 100 Gb
devoted to the parity information. Surrendering 25% of your capacity for
security is a nuisance, but one I happily pay. And it beats the 50% capacity
loss of RAID 1.

There are many other RAID "levels", some standard and some non-standard. 
  You can read more about it here:
   http://en.wikipedia.org/wiki/RAID

For my money, RAID 5 is the way to go.

Also, FWIW, it is true that a lightning hit is probably going to wipe out
all the data that aren't stored completely offline or in a remote location.
But in my 29 years or so of personal computing, I have never lost a drive to
lightning (even though for the last decade or so I have had a lot of
aluminum pointed at the sky with coaxial cable bringing the captured energy
right into my house), but I've lost at least a dozen hard drives to drive
failures of various other sorts, such as head crashes, bearing seizure, etc.
Now, I'm not discounting the possibility of lightning-induced data loss, as
I personally know several hams whose homes have suffered direct or
near-direct hits and who lost virtually everything electronic.  I'm just
saying that drive failure from other causes is much more likely.

So, for my money, a RAID 5 array is a useful solution, but anything critical
still gets backed up elsewhere. In the old days, that was floppy disks, but
my first hard drive was a 5 megabyte model, which became a 3.75 Mb hard
drive when one of the 4 heads crashed. So, backing up onto 100 Kb floppies
(yes, they really WERE that low in capacity!) was doable. Later, as the hard
drives became larger, I tried tape drives, 90 Mb and 230 Mb Bernoulli
drives, 120 Mb LS-120 "Super Floppies", 2 Gb Jaz drives, 700 Mb
CD-recordable drives, etc. Nowadays, most important stuff gets burned onto a
4.7 Gb DVD-recordable disc, or a
9.4 Gb dual-layer recordable DVD.

If you are contemplating a system with backup drives that are externally
mounted, keep in mind that a USB (480 Mbps), FireWire (400 Mbps) or FireWire
2 (800 Mbps) interface is reasonably fast, but no match for the intrinsic
speed of SATA (1500 Mbps) or SATA-2 (3000 Mbps).  But if your computer
doesn't have eSATA connectors, you will have to go with a slower interface.
And if your PC has a PCI bus, you can't plug in a PCI card with eSATA ports
that run at full speed, since the PCI bus can't keep up. You'd need a PCI
Express card to keep up.  For my external RAID
5 array, I chose one with a Gigabit Ethernet interface. It runs at full
speed with my desktop PC or my high-end laptop, both of which have Gigabit
Ethernet built in, but it will also run at 100 Mbps if plugged into a
computer with 100-Base T Ethernet only.

73 de W0JT
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