I recommend reading the guidelines found in the 'Virtual Provisioning' and 'FAST Virtual Provisioning' sections of EMC CLARiiON Best Practices for Performance and Availability, FLARE 30.0, which still applies to pools.  Generally, pools should be created and expanded in 'RAID group increments'.  The ideal increment size is the default standard for the pool's RAID level.  So for example, in a RAID-level 5 pool, the default RAID group is a 4+1.  The ideal pool size or tier size is a multiple of 5-drives.  To further the example, using the 80/20 rule of thumb to create a small two-tiered pool, you'd use 25-drives (5x 15K rpm SAS, and 20x NL-SAS).  The more information you have about your workload, the finer your provisioning can get.  However, that topmost tier being setup as a 4+1 is giving you the best performance and capacity utilization.  Ideally the OP should have 10x NL-SAS drives for the mass storage tier.  That would be 2x (4+1)s.  The trick with having an odd number of drives with respect to the default is to get 'symmetrical RAID groups' created in the pool.  (This is explained in Best Practices.)  The NL-SAS drives need to be created as 2x (3+1)s or a single (6+1) (use the remaining drive as a hot spare) to get the better performance with 8x NL-SAS drives.  Any expansion of the tiered pool, that has to do with the normal increase of data, would mean adding RAID group increments of drives to the NL-SAS tier.  If you are adding additional workloads, you'd expand all the tiers by the original ratio of drives.  There is a bigger performance advantage to using available flash drives as a FAST Cache over creating triple-tiered pools,   This is especially true when you only have a very few flash drives.  The few available flash drives are used more effectively as a secondary cache.  I also recommend reading the EMC VNX Virtual Provisioning whitepaper.  It contains further examples for creating both homogenous and heterogeneous (FAST VP) pools.  I hope this helps.
storage pool can have only one RAID, can i initial a RAID 5 pool with 3 SSD, 5 SAS and 8 NL-SAS ? when i extend the pool, is there any rule for added disk number? shoud it be integer times of disk type (e.g. 3, 5, 8 or 6, 10, 16)or any disk number can be accepted?
RAID of storage pool