Upon upgrading the first host in production (restbase1007) to Cassandra 2.2.6, very high levels of disk read throughput were encountered (10x or more). Ultimately it required setting disk_access_mode to mmap_index_only to restore normal levels. Since memory-mapped decompression reads were an important new feature of 2.2 for us, we should figure out why this, and what is needed to correct it.
This issue still affects Cassandra 3, so it is still relevant. The work-around from 2.2 still works, but he option might no longer be officially supported (@Eevans: is this accurate?).