Bluestore/RocksDB will only put the next level up size of DB on flash if the whole size will fit. These sizes are roughly 3GB,30GB,300GB. Anything in-between those sizes are pointless. Only ~3GB of SSD will ever be used out of a 28GB partition. Likewise a 240GB partition is also pointless as only ~30GB will be used.
How do I find the right SSD/NVMe partition size for hot DB.