Spare rules  These rules for the numbers and types of spares required for each Symmetrix VMAX system are designed to minimize risk by providing multiple spares in various locations for each type of data drive configured. The system’s current configuration as well as expansion potential is taken into account. Spares must be configured for each of the system’s drive types (capacity, speed, and block size). The spare rules are considered to be the minimum, and additional spares can be configured. Spare drives on every loop for every type of data drive on the loop gives the best possible Permanent Sparing coverage. Direct Sparing makes use of temporary relaxation of these stringent guidelines allowing full spare coverage with fewer spares.  HDD Sparing Rules  Spare drives are required for every Symmetrix VMAX system. The configuration program residing on the service processor prevents the creation of Symmetrix VMAX configurations that do not contain the required numbers and types of spare drives. A valid pool of spares for each drive type must be available.  The amount and types of spares required for Symmetrix VMAX systems are calculated as follows: 
May I know what's the VMAX spare drives recommended configuration? What is minimum number of spare drives for VMAX array? 
VMAX spare drive configuration