ZFS can display I/O statistics for a given pool using the iostat subcommand.
By default, it will display one line of current activity. To display stats
every 5 seconds run the following command (cancel with CTRL+C):

zpool iostat 5

To view individual disk activities, specify the -v parameter:

zpool iostat -v

Of course, both can be combined. For more options, see zpool(8).
-- Benedict Reuschling <bcr@FreeBSD.org>