X-RAID Volume Smaller than expected

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Mauser69 wrote:

My RN214 was configured with 2x8 TB + 2x1.5 TB drives, and it reported a total volume size of 9.9 TB. 

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Depends widely again on how this set-up came together. If this was a scratch install, StephenB theory is appropriate.  If this X-RAID started life with two 1.5 TB drives only, StephenB theory is appropriate again as the 2*1.5TB RAID1 will become a 4*1.5 RAID5 plus a 2*6.5 TB RAID1. 

If this started from two 8 TB drives and two 1.5 TB units were added later, the two 8 TB drives will be RAID1, and the two added 1.5 TB units will be added without re-partitioning the existing 2*8 TB RAID1.

Essentially the same what happens when replacing the 1.5 TB by two 3 TB units - the existing 1.5 TB partition on the 8 TB drives won't change, you end with with a RAID1 from 2*6.5TB, a RAID5 from 1.5TB, and a RAID1 from the "new" 2*1.5TB. As this is mostly concatenate the performance advantage by the RAIDs is mixed, writing will happen for a good part on a single disk performance minus some overhead for the RAID1, read can happen from two storage blocks in parallel.

Shoot me StephenB - you know as a QNAP geek I prefer strict RAID configs without mixing RAID1 and RAID5 ... and I'm aware of the risks of running RAID5 starting with large capacity drives - where RAID6 is mandatory but X-RAID does ignore this. So I can be bluntly wrong in my understanding of X-RAID, at least with four storage blocks.

In no way the additional TBs added will add 100% to the available capacity.