"RAID 6, Fast" will always be full and data will be pushed in and out of that "reserved" capacity during Data Progression. If there is unallocated diskspace in that tier then the system will add more space to the Standard pool as needed. The R10 and R5/6 pools expand and contract automatically based on the usage of the SAN.  If you have a need for lots of space quickly in a specific tier (like purposely injesting a new file server to Tier 3 and then letting new writes hit Tier 1 then progress down) you can contact Copilot and they can assist with manually expanding or contracting the RAID pools.   In general, you can always contact Copilot Support to discuss these things and they can give you an assessed health of the RAID striping and pool capacities. They're more than just break fix!
Looking at my "Storage Types", I see my two sub-tiers in Tier3 are soon to run out of diskspace. My "Raid 6, Fast" and "Raid 6, Standard" are soon to run out. My question is, will the Compellent the utilize the diskspace allocated to the Raid 10 subtiers in the same tier instead?Or will the compellent system automatically allocate more blocks as needed? (I have a lot of unallocated diskspace at the moment)  Or how should I go around this?
Sub-Tiers in Compellent