RHEL to FreeBSD file server

[Bob Friesenhahn]

On Mon, 12 Nov 2012, kpneal at pobox.com wrote:
>
> With your setup of 11 mirrors you have a good mixture of read and write
> performance, but you've compromised on the safety. The reason that RAID 6
> (and thus raidz2) and up were invented was because drives that get used
> together tend to fail together. If you lose a drive in a mirror there is
> an elevated probability that the replacement drive will not be in place
> before the remaining leg of the mirror fails. If that happens then you've
> lost the pool. (Drive failures are _not_ independent.)

Do you have a reference to independent data which supports this claim 
that drive failures are not independent?  The whole function of RAID 
assumes that drive failures are independent.

If drives share a chassis, care should be taken to make sure that 
redundant drives are not in physical proximity to each other and that 
they are supported via a different controller, I/O path, and power 
supply.  If the drives are in a different chassis then their failures 
should be completely independent outside of a shared event like power 
surge, fire, EMP, flood, or sun-spot activity.

The idea of raidz2 vdevs of four drives each sounds nice but will 
suffer from decreased performance and increased time to replace a 
failed disk.   There are always tradeoffs.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/