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RAID 1+0: Striping a Pair of Mirrors for Fault Tolerance

See the overview of the OWC Mercury Elite Pro Dual.

With two OWC Mercury Elite Pro Dual units on hand (briefly), a thought arose: daisy chained and using one Thunderbolt port, how about an easy-as-pie fault tolerant setup?

Fault tolerance

Some workflow environments have a “cannot be down” operating principle. Examples might include a service bureau, where jobs have to be finished on some deadline (e.g., by 5PM).

Fault tolerance means some degree of failure can be tolerated with no data loss and no loss of functionality. A RAID-1 mirror is the most basic kind of fault tolerance, but sometimes more performance is desired. Thus, a RAID-0 stripe of RAID-1 mirrors comes to mind.

With a pair of the OWC Mercury Elite Pro Dual units, it is easy to make a striped pair of mirrors (RAID 1+0), which achieves both high performance and fault tolerance for critical workflow environments:

  1. Configure each OWC Mercury Elite Pro Dual unit to Mirror mode.
  2. Make a RAID-0 stripe of the two units.

With this setup, one drive can fail in each unit with no data loss and continued functionality (replaced the failed drive ASAP).

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Performance

These two units were mismatched but were what was on hand; one had dual 4TB drives and the other had dual 3TB drives. Faster results might be expected from matched and/or faster hard drives, not that these results are at all slow!

Testing performed with the DiskTester fill-volume command. DiskTester is part of diglloydTools. The command line version was used here.

llcR:~ lloyd$ disktester run-sequential -i 10 r1+0
DiskTester 2.2 64-bit, diglloydTools 2.2.0, 2012-12-15 18:05
OS X 10.9.1, 8 CPU cores, 16384MB memory
Allocating maximum size contiguous file on "r1+0" (2TB)...1.99TB (99.5% of volume size)
Using test size of 4GB, 4MB at a time at start (0%), within a 1.99TB test file.
...elided...
Monday, December 30, 2013 at 20:37:25  Pacific Standard Time, volume "r1+0" (2TB)  
--------------- Averages for "r1+0" (4GB/4MB, 10 iterations) ---------------
Iteration 	Write MB/sec	Read MB/sec
1    	     318    	    318    
2    	     302    	    317    
3    	     316    	    317    
4    	     316    	    317    
5    	     319    	    316    
6    	     319    	    316    
7    	     318    	    316    
8    	     318    	    317    
9    	     316    	    318    
10    	     317    	    319    
Slowest  	     302    	    316    
Fastest  	     319    	    319    
Average  	     316    	    317    
Median  	     317    	    317    
Range   	    17.2    	    3.22   
Command "run-sequential" executed in 261.02 seconds on Monday, December 30, 2013 at 20:41:46  Pacific Standard Time
Pair of OWC Mercury Elite Pro Dual units, each with two drives

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