When assigning storage policies, and assigning storage to data nodes, dfs.datanode.data.dir is updated in hdfs-site.xml. Does this mean that all drives on the data node should be the same. I.e. best practice is to not mix drive types on a node, or is there additional granularity that can be applied to allow multiple storage types on the node.
It is perfectly fine to mix drive types on a node. Just tag the storage type ([SSD]/[DISK]/[ARCHIVE]/[RAM_DISK]) for each directory specified in dfs.datanode.data.dir; see also https://hadoop.apache.org/docs/r2.8.2/hadoop-project-dist/hadoop-hdfs/hdfs-default.xml (search "dfs.datanode.data.dir")