fio --direct=1 --rw=write --ioengine=libaio --runtime=300 --bs=4k --numjobs=4 --iodepth=32 --size=2G --name=4096-direct --filename=/dev/vdb --group_reporting

This is my "dd" test for any storage system. Filename can be a regular file; you can also pass --directory instead. I would love to see numbers for both this test as well as with --bs=4M.

Also experiment with --rw=rw|randrw|read :)

(and make sure your device is large enough to hold numjobs*size)