Home Is Home: The Physical Practicalities - Smart Home Automation with Linux and Raspberry Pi - page 133

Reducing the load is something that should be considered in as many cases as possible because, although you
might save only a watt, over the course of a year this can add up to a reasonable savings.

Server Coordination
Having multiple servers to control your house is fine. But if they're all switched on all the time, your electricity bill will
increase disproportionately to the benefit gained. This also doesn't consider the case where the various machines
can't communicate because the master server is offline.
Coordinating how to switch specific machines on and off to save power is an easy problem . . . for a human. But
it's incredibly difficult for a machine for automate! You would need to know what services are running on which
machines, what dependencies exist on other hardware, and so on. This information would need to be synchronized
among all machines in case one went offline (because of power outages or loss of network connectivity).
Consequently, it is better to not try or to do one of the following:

u Have human-designated time cycles for each machine

u Expect a larger than usual electricity bill

When running a separate media server to transcode streams to the MVPMC, for example, you will know that
the machine needs to be on from 6 p.m. each weekday and all weekend, for example. If the machine is also in charge
of recording TV programs, you will have write custom code to prevent it from switching off during one of those
recordings.
The inverse timetable is true of a machine intended to be a download server, when you only want it to be leeching
your bandwidth during the early hours of the morning when you're not likely to be web surfing or working from home.
Although it is possible for a machine to self-terminate (through the Linux command shutdown -h now ), you can't
generally force it to wake up at a specific time. Therefore, you will always need one machine (such as the lowest-power
device you have) that remains on 24/7 to coordinate all the others.
Having a machine switch on automatically requires a technology called Wake on LAN (WOL). WOL is enabled on
many machines (although sometimes defaulted to off in the BIOS) whereby the motherboard watches for a specially
formatted network message containing a “magic packet” sent directly to the machine in question. Since there is no IP
address for a machine that is powered down, the MAC address must always be used. (It's labeled as HWaddr from an
ifconfig command.)
The command and package to initiate this magic is as follows:

etherwake 00:1d:33:a5:63:16

Note that this works only from a “soft” power down and needs power available to it, along with that of any routers
between the sending machine and the target. That's an important point to remember during a power cut! Also, note
that the Raspberry Pi doesn't have a soft power down option—it's without a on/off switch, so it's either on or off.
However, it has such a low current draw that it isn't worth worrying about.

N Sometimes a password is required for a WOL command to be accepted. This is set in the BIOS of the particular
machine and passed as an argument to etherwake .

Note

Switching particular machines off is easier, because it's something most desktop users do every day, so I'll
spend little time on it here. In its traditional invocations, you can initiate a shutdown with a single command, with
a given delay from the issue of the command to the action with any console alert message you choose. You can also
countermand any unenacted shutdown command with the -c flag.