To be able to trully test the performance of the drives in a NAS I need a proper tool that runs on the NAS unit itself. I’ve seen a lot bench-marking done with bonnie++ but installing it in FreeNAS wasn’t straightforward.
First you need to install a jail, then you compile the program from source. FreeNAS is set up quite nicely to do this, so it’s not too much trouble, but figuring out this info took some time.
Some links for future reference:
https://blog.pivotal.io/labs/labs/high-performing-mid-range-nas-server
https://doc.freenas.org/9.3/freenas_jails.html
http://louwrentius.com/74tb-diy-nas-based-on-zfs-on-linux.html