How can I benchmark disk throughput using dd? (writing to /dev/null is instantaneous?)

I'm trying to perform a simple disk benchmark (throughput) of my ZFS filesystem using dd. Here is my command:

dd if=/tank/media/video/largefile.mp4 of=/dev/null bs=1M count=1000

However, it finishes instanteously:

+0 records in
1000+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.18307 s, 5.7 GB/s

This is obviously not actually reading the file because that speed is unrealistic. How can I perform a read-only test for throughput? I don't want to write the file anywhere because then I'm also testing write speed. Also, what is a typical blocksize to use? (for example, to simulate copying over a network using SMB or similar)