Building a NAS, part 5: minimizing power consumption

My plan is to let my NAS run 24/7 if the impact on my electricity bill is acceptable. To measure power consumption, I have purchased a power consumption meter that you can plug in between a wall socket and some device. It is one of those tools that can provide hours of quality geek entertainment. So much devices to measure around the house, so little time! :-)

The following table shows the passive power consumption of my NAS after each round of power saving measures. With "passive", I mean that the NAS was not doing anything useful like reading from or writing to the ZFS file systems.

Configuration	power consumption (watts)
initial	80
under-clocked CPU	69
removed AGP video card	59
removed PATA CD-ROM drive	57
removed unused eSATA RAID1 PCI card	55
2 storage disks in standby	47

Take it slow

The Athlon CPU and/or the mother board in this box is apparently too old to support dynamic cpu frequency scaling. It also doesn't seem to support the AMD power saving mode which you can control with the athcool package. Instead, I had to get down and dirty with the BIOS settings and set the CPU multiplier to the minimum value. According to cat /proc/cpuinfo, this slowed down the CPU from 2GHz to 1GHz. Profit: -11 watts.

Bare necessities

I had to connect a video card and a CD-ROM drive to install debian Lenny. Now that the server is running and connected to the network, I can throw those out again and manage the system remotely. Fortunately the BIOS supports booting without a video card. I also removed a PCI card for eSATA RAID1 which I initially thought I would need. Profit: -14 watts

Spin down those storage disks

With the hdparm command we can inspect the power state of a disk and configure stand-by mode:

aptitude install hdparm
hdparm -C /dev/sda # prints "active/idle"
# now tell disk to go to stand-by whenever not used for 2min
# note the very strange S-value to time mapping; consult man hdparm!
hdparm -S 25 /dev/sda
sleep 120
hdparm -C /dev/sda # should print "stand-by"

To make the power saving configuration permanent, I added this to /etc/hdparm.conf. Note the use of /dev/disk/by-id to keep the settings correct even if we start changing the NAS hardware:

/dev/disk/by-id/scsi-SATA_ST3500418AS_9VM7RWGV {
spindown_time = 25
}

/dev/disk/by-id/scsi-SATA_ST3500418AS_9VM7SHA5 {
spindown_time = 25
}

Profit: -8 watts

Leave the system disk alone

It is much harder to get power savings for the system disk, because it is used all the time for logging and by a bunch of daemons. Trying to put this disk in stand-by will just cause it to frequently spin down and up again.

Instead, I'm using a disk recovered from a dead laptop as the system drive. This requires a cheap 2.5" to 3.5" IDE converter cable. Such a drive is already extremely efficient; the potential savings of putting it in stand-by are negligible (~1 watt).

Cost before and after

( update: corrected kwh cost, my original estimate was about 50% of actual cost because I based it solely on Electrabel's power generation cost, and forgot the distribution costs on my bill - thank you Peter!)

At 80 watt, energy consumption per year was 80 watt * (24 * 365) hours = 700.8 kwh. At an average 0.16 euro/kwh that will cost 112 euros. I have reduced that to 66 euros. This goes to show that a 20 euro energy consumption meter can yield a return on investment rather fast.

Another lesson I'll be remembering from these calculations: for my current contract, I can estimate the yearly cost for the continuous consumption of a device as 1.4 euro per watt.