Well Mitch don't be afraid if you have FAST/VP.   The document that (actually i downloaded today) Mr. Code provide is very good and you have to read it/study it carefully. It is very informative.   Now regarding the tiers and the capacity, it is ok. It depends on what appications you are using. Actually i believe that if you have OLTP appications the 2TB will be consumed immediatelly, but it is ok. For the 40TB of FC, it depends on your needs and your applications. Why don't you use and SATA or SAS as well.  As i told you, if you have Oracle databases, you can provide devices for datafiles and you can fully pre-allocate those devices. So with that move you know in any time that you have devices that already consume the space and you won't fear. I know what you are saying. To ave a Full Pool is a very bad thing, but believe me you can avoid it. Also there is the option to epxand the Thin Pools with new physical disks and TDATs.  Don't be afraid and try to monitor everything and this will help you a very good reporting and monitoring tool such as SRM Suite. If you don't have money to spend now for that Suite, then try through UniSphere to monitor your thin pools.    Some reasons as well to use FAST:     The primary benefits of FAST include:  ·         Improving application performance at the same cost, or providing the same application performance at lower cost. Cost is defined as: acquisition (both hardware and software), space/energy, and management expense.  ·         Elimination of manually tiering applications when workload characteristics change over time.  ·         Automating the process of identifying data that can benefit from Enterprise Flash Drives (EFDs) or that can be kept on higher-capacity, less-expensive SATA drives without impacting performance.  ·         Optimizing and prioritizing business applications, allowing customers to dynamically allocate storage resources within a single array configuration.  ·         Delivering greater flexibility in meeting different price/performance ratios throughout the lifecycle of the stored information.    The need for FAST  Due to advances in drive technology, and the need for storage consolidation, the number of drive types supported by Symmetrix arrays has grown significantly. These drives span a range of storage-service specializations and cost characteristics that differ greatly.  Several differences exist between the drive types supported by the 4  Fully Automated Storage Tiering  Implementing FAST VP for EMC Symmetrix VMAX Series Arrays Technical Notes    Symmetrix VMAX Family arrays. The primary differences are:  ·         Response time  ·         Cost per unit of storage capacity  ·         Cost per unit of storage request processing   
I'd like to know what kind of subscription levels you folks feel comfortable with in your thin pools.  Myself, I'd like to stay as close to 100% or lower as possible, but I've seen 120%! 
Subscription Levels in Thin Pools