Try this article. It explains in explicit detail how storage spaces work to optimize speed and recoverability. IT IS NOT JBOD.
To explain here, Storage spaces stripes your data across all applicable drives evenly, filling them up evenly.
If you are working in a Mirror system, all data is replicated exactly with its paired mirror drive. so if you have 2 drive in mirror, 1 and 3 will be identical, and 2 and 4 will be identical.
If you are working in Parity mode, the last drive in the array is flagged as parity and stores all that information. the remaining drives will stripe data across them evenly.
It does not make any optimizations based on file access frequency or medium. Since storage spaces cant be booted to, it assumes that none of the files stored therein are system files and treats all files equally. (this is the same behavior as if you were using a traditional RAID array, so this is NOT a bug.)