Users need to have SSDs and HDDs in the same storage pool in order to do "tier storage".
When you create a virtual hard disk with tiered storage, Windows allows you to specify the amount of traditional hard-disk space and the amount of solid-state storage space that you want to use for the new virtual hard disk. If you create a fast tier that is more than 1 GB in size Windows will use 1 GB of SSD space as a write-back cache. This write back cache improves performance by intercepting and smoothing out write operations.
This advantage is extremely powerful when dealing with random read workloads and therefore, improvement of overall performance. According to our testing results, however, we suggest our customer configure the write back cache at size 1GB. That's because if you configure the write back cache as 10G, we found the performance improvement isn't worth the capacity that SSD has sacrificed.