How many SP do you have? How many SDSs do you have? 3? and in each SDS 3 drives?  As you can see in the output the spare is configured to 34% (rounded up from 33.33334). Spare is calculated to be able to protect "loosing" 1 fault unit, if they are all even (Basically, your largest fault unit). Fault unit can be a single SDS, or a Fault Set (FS) made out of several SDSs. So in you case 16.4 TB * 34/100 = 5.6 TB spare capacity (Blue section)  You created 2 * ~2 TB thick volumes = ~4 TB (but due to the RAID1 it's actually ~8 TB (Green section) So what is left is 3 TB available for volume allocation, but again, this needs to be divided to 2 (RAID1), so you have only 1.5 TB available for volume allocation (Grey section)  This is why you can have better disk space utilization to create a 30 TB storage made of 10 nodes/SDS (in that case if they are all even spare will be only 10%), rather than 3 nodes. And as a bonus, you will get better performance  due to ScaleIO's parallel disk utilization across all SDSs.
[IMAGE] This is my capacity. I have 9 drives of ~2TB each. I understand that 16.4 is about what it comes out to. In a raid 1 situation, I see I have roughly 7.8 TB of usable space. Can someone explain what the 3TB of unused space is there?  Also, I only have 2 2TB volumes created and mapped to SDCs, Why is it that I only have 1.5TB Free?  Thanks!  torage Pool SP1 (Id: xxxxxxxxxx) has 2 volumes and 1.5 TB (1520 GB) available for volume allocation   The number of parallel rebuild/rebalance jobs: 2   Rebuild is enabled and using Limit-Concurrent-IO policy with the following parameters:   Number of concurrent IOs per device: 1   Rebalance is enabled and using Favor-Application-IO policy with the following parameters:   Number of concurrent IOs per device: 1, Bandwidth limit per device: 10240 KB per second   Background device scanner: Disabled   Zero padding is enabled   Spare policy: 34% out of total   Uses RAM Read Cache   RAM Read Cache write handling mode is 'cached'    16.4 TB (16754 GB) total capacity   3.0 TB (3057 GB) unused capacity   0 Bytes snapshots capacity   7.8 TB (8000 GB) in-use capacity   0 Bytes thin capacity   7.8 TB (8000 GB) protected capacity   0 Bytes failed capacity   0 Bytes degraded-failed capacity   0 Bytes degraded-healthy capacity   0 Bytes unreachable-unused capacity   0 Bytes active rebalance capacity   0 Bytes pending rebalance capacity   0 Bytes active fwd-rebuild capacity   0 Bytes pending fwd-rebuild capacity   0 Bytes active bck-rebuild capacity   0 Bytes pending bck-rebuild capacity   0 Bytes rebalance capacity   0 Bytes fwd-rebuild capacity   0 Bytes bck-rebuild capacity   0 Bytes active moving capacity   0 Bytes pending moving capacity   0 Bytes total moving capacity   5.6 TB (5696 GB) spare capacity   7.8 TB (8000 GB) at-rest capacity   0 Bytes decreased capacity    Primary-reads                            1 IOPS 1.6 KB (1638 Bytes) per-second   Primary-writes                           40 IOPS 238.6 KB (244326 Bytes) per-second   Secondary-reads                          0 IOPS 0 Bytes per-second   Secondary-writes                         43 IOPS 232.0 KB (237568 Bytes) per-second   Backward-rebuild-reads                   0 IOPS 0 Bytes per-second   Backward-rebuild-writes                  0 IOPS 0 Bytes per-second   Forward-rebuild-reads                    0 IOPS 0 Bytes per-second   Forward-rebuild-writes                   0 IOPS 0 Bytes per-second   Rebalance-reads                          0 IOPS 0 Bytes per-second   Rebalance-writes                         0 IOPS 0 Bytes per-second  Volumes summary:   2 thick-provisioned volumes. Total size: 3.9 TB (4000 GB)
Can somebody explain how capacity works in ScaleIO?