You need to arrange to have the command run at boot time.
In the past, you could do this by placing it in the file
/etc/rc.local. You can still do this, if you wish. Just create the file (as it does not exist by default anymore) and it will be run on next boot.
You can also simply create a new systemd oneshot unit which runs your desired command at boot time. For example:
Description=Set readahead for the hard drive device
ExecStart=/sbin/blockdev --setra 8192 /dev/sda
You might use this latter method if you expect admins of this server to not be familiar with the old rc.local script.