To cache the same amount of data as a 512GB Flash Cache card can hold, you will need four 200GB SSDs for data plus one parity drive and one hot spare.
In the industry, SSD and HDD capacities are stated in decimal (base10) values; the 512GB Flash Cache card capacity is a binary (base2) value, which is 7% larger. In addition, cache metadata for Flash Pool is stored on the SSDs whereas Flash Cache metadata is kept in controller main memory.
For more information about Flash Pool, please see TR-4070 Flash Pool Design and Implementation Guide: http://www.netapp.com/us/media/tr-4070.pdf
Skip Shapiro
NetApp FAS Flash TME