I found it worth noting that the cache is primarily interested through linux's built in software raid system, md. SSDs in raid0 (strip), persistent disk in raid1 (mirror).
Not just a mirror but a specific setup of the mirror where it does its best to only write to one of the mirror devices, unless the other starts reporting read errors. This way they don't lose data, errors get handled gracefully and they can still snapshot the high latency disks for replication/migration tasks. That write-mostly feature was not something I was aware existed but it sounds absolutely perfect for a lot of use cases.
