Well, in my opinion nothing is completely opaque or transparent, but to me ORMs (given their ability to fall back to arbitrary SQL, which their main task is to

Well, in my opinion nothing is completely opaque or transparent, but to me ORMs (given their ability to fall back to arbitrary SQL, which their main task is to abstract away) are way more transparent than relational databases (that constrain you to their relational model). Indeed, RDBMSes give you a myriad parameters to tune, but if you think about it almost all of them affect the behavior of the system in a global way and almost none of them is really an escape hatch that lets you do things in your non-relational non-ACID way (e.g. you can't choose not to persist some of your writes to a given table immediately, although you can choose to keep the entire table in a memory-based storage engine in mysql). SSDs are not that different from hdds I believe. The nature of hard disks is that you should minimize random access. With SSDs you can do all random access you want on reads, but you still better write continuous blocks. There are storage systems like Cassandra that were developed for hdds but conform well to this SSD-optimal pattern, it would be interesting to see how they perform on SSDs.