DESCRIPTION:            

Creating a Raid-1 (mirror) out of an odd number of disks worked. How?

The T3 mirroring algorithm was written to accommodate mirrors with 
odd-numbers of disks.  The intent of this document is to explain how 
that works.

CREATING THE VOLUME:

Here is how one can create, via the CLI, a 3-disk RAID-1 (mirror).
Each of the disks used here are 18GB.

   t3a:/:<10> vol add v1 data u1d1-3 raid 1
 
   t3a:/:<11> vol init v1 data
   WARNING - Volume data will be initialized to zero.
   WARNING - Volume initialization can take a significant amount of time.
   Continue ? [N]: Y

   t3a:/:<12> vol list
   volume        capacity   raid   data     standby
   v1             26.8 GB    1     u1d1-3    none

Note that a 3-disk mirror of 18GB (actually 17.9GB) disks yields 26.8GB
of usable space.  That's 17.9GB times 3, then divided by 2 because the
data is mirrored.

	17.9 x 3
	--------  =  26.8
	    2


SO, HOW DOES THIS WORK?  WHAT'S REALLY GOING ON?

Assuming a 3-disk mirror, when one writes 6 blocks of data to the
mirror LUN, the 6 blocks (and likewise, subsequent blocks) will be
arranged on the 3 disks as shown below.  The "D" refers to data, and
the "M" refers to mirror.

With the data arranged in this fashion, it is easy to see that if a
disk fails, the data will still be available.

               disk-1  disk-2  disk-3
             +-------------------------
   Data ->   |  blk1    blk2    blk3
   Mirror -> |  blk3    blk1    blk2
   Data ->   |  blk4    blk5    blk6
   Mirror -> |  blk6    blk4    blk5