Here goes... Currently we have servers with 7 disks of ES data. Each datanode gets a data.path list of all 7 local disks. With ES 1.7 data for a single shard is spread over all disks using 'least used'. With 2.0 data for a single shard will be put on 1 disk, not all 7. Great for resiliency, because it solves 'partially failed shards' because 1 of 7 disks died.
But ... having data for 1 shard all on 1 disk, makes that disk hot while indexing. Much hotter than with 1.7, because that version spreads write ops over all disks. So with ES 2.0 we are thinking spinning disks will not work anymore.
And so we are investigating alternatives, except throwing in flash storage...