On 4/27/2011 3:18 PM, Jeffrey J. Kosowsky wrote:
>
>   >  ie, 2 RAID1 devices are always installed, another 3 devices are rotated,
>   >  with one offsite, one onsite, and the third in one of (transit, offsite,
>   >  onsite).
> Well, this is the first anyone mentioned about "another 3 devices are
> rotated". Of course, if you are allowed to posit lots of other offline
> backups then be definition you risk less even if all of your current
> RAID devices fail. The OP however seemed to imply a *single* 3rd
> device not 3 other devices...

Rotating some number (>1) of devices in that 3rd slot seems like a 
reasonable assumption. The point is to have an offsite copy.  You can't 
do that if your only extra instance is in the same building, hooked to 
the same computer.

> I think what people don't fully realize is that no matter how good
> your software RAID is, a hardware problem can easily result in an
> error propagating across all live RAID1 copies... as I discovered,
> this is a very real and painful case...

They don't realize it because it shouldn't be related to RAID operation 
except that a read failure on the source also leaves the sync target 
corrupted.   That said, any of the site disasters that your offsite copy 
is intended to protect against can happen any time your copies are all 
in the same location.  So don't do that.  And the same applies for 
anyone trying to do live network copies of the archive to a single 
remote instance.  Most of the ways you could do that will leave your 
copy corrupted if the source dies midway through the operation.

>   >  Thus you need to lose both RAID1
>   >  devices during a resync before you need to rely on the device you just
>   >  removed, or finally the offsite copy.
>
> Which is *precisely* what I was proposing except that in addition to
> failing the device, I suggested removing it physically for extra
> security (again assuming you don't have 3 other backup devices
> rotating around offsite :P)

No, you really do need additional copies in the rotation so they are 
never all in the same place when the fire/flood/earthquake/lightning (or 
raid-weirdness) happens.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com

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