When planning your disk layout, the rule of thumb is to distribute the SSDs evenly across the busses in the lowest enclosure number and lowest drive slot number where possible.  When it comes to planning for future expansion, it can often be a good idea to leave some empty slots in DAEs for SSD expansion. Typically, the SSDs in a VNX will be deployed as RAID-5 (4+1), excluding any FAST Cache SSDs which will be deployed as RAID-1.  Leaving five (5) empty slots in the DAEs that are the first in each bus, excluding bus0, enclosure0 which will have the Vault drives in it. If there are only a few back-end busses available, then leave five (5) empty slots in each DAE, until you have enough free/empty drive slots to cater for the future SSD expansion.
We are purchasing a VNX with some SSDs, but we know that we will be purchasing additional SSDs in the future.  What should we do to plan for the additional SSD upgrades in the future?
How to plan for future SSD upgrades in a VNX