Yes you are missing something.  One volume on 236GB 6+2 volume will create 8 physical hypers on 8 drives.  Each drive will have a ~39.3GB split or hyper on it (39.3 x 6 = 235.8 usable capacity)  The two parity overheads don't count for the usable capacity, but take up space on the 8 drives.
Hi all  from the document Simplified VMAX Blueprint Guide   VMAX + 5874 & 75 EMC Enginuity™ Solutions Enabler v7 which is in EMC speed    it said to create TDAT devices with 259912 cyl = 236.49 GB on 1000GB SATA drive with Raid6(6+2)   while other document direct us to have at least 8 hypers per disk     if i create 236.49 TDAT on 1000 GB R6(6+2) i will end up with < 4 hypers perdisk   so what the best practics on that, the same goes for other disk sizes and disk groups, the documents contradict each other. I am looking for the best perofrmance for an environment based completley on Thin provisoning and FAST VP. I got this from the document mentioned:    for 1TB drives please use the following TDAT device size: RAID6 (6+2) device size = 259912 CYL = 237.96 GB total devices per group 23 --------------------------------- for 450GB Drives RAID 6 (6+2) Device size= 260570 CYL =238.56 GB total devices per group 10 ------------------- for 450GB Drives RAID 1 (2 drives) Device size= 54282 CYL= 49.70 GB total devices per group 8 ------------------ For 300 GB RAID 1 (2 drives) Device size =35960 CYL = 32.92 GB Total devices  per group 8 ----------------------         
TDAT Device sizes for maximum performance