ioping measures the latency of individual I/O requests, the way ping does for a network path. It is a fast check of whether a disk or network filesystem is responsive right now.
sudo dnf install ioping # or apt-get install ioping
ioping -c 10 . # 10 requests against the current directory's filesystem
sudo ioping -c 10 /dev/sda # raw device read latency
ioping -R /dev/sda # seek rate test (requests per second)
ioping -RL /dev/sda # sequential (-L) rate test
ioping -S 512M -W /data # write test: 512 MB working set (writes to a temp file on /data)
-W (write) tests on a raw device would destroy data. Point it at a directory or file instead. For load-style benchmarking use fio.