SSD Cache Basic
SSD cache helps accessing same set of files frequently. But if the system just holding media files, most likely they won't be visited again, then cache doesn't help.
RAM is required for SSD cache, but adding RAM is more directly impact the perform, because there is no duplication between harddisk and SSD.
The required amount of RAM is calculated before cache created.
One SSD disk only can be configured for one volume in read-only mode.
Two or more SSD disks can be configured as one raid, for one volume in read-write mode.
Currently, one SSD disk or raid can not be partitioned for different volume.
Others suggest 64GB or more of RAM before adding a cache, otherwise, will slow the system down if add a cache with 16GB RAM.
Fusion disk could be another choise because the SSD can be used as storage as well, no waste of space.