Genuinely curious: where would one expect the drive performance to fluctuate? Wear ? Lack of TRIM ? Some form of timely GC process on disk firmware ? Fragmentation or compaction of some sort ? Maybe weird shenanigans with RAID setups with disks from different vendors and batches ?

Embarking right now on a long-term embedded storage project and wondering what people actually monitor (apart from SMART and latency/throughput at app or db-level).

It could be regular, like SQL "analyze table", it could be one-off. The point is that it would be an automatic tool.