Something is going wrong - How can the optimal_io_size be smaller than the minimum_io_size:

root@host:~# cat /sys/block/sdb/alignment_offset
0
root@host:~# cat /sys/block/sdb/queue/optimal_io_size
655360
root@host:~# cat /sys/block/sdb/queue/minimum_io_size
2097152

This is open-iscsi connected to an Synology LUN.