Raspberry Tau: a Pi cluster

The Raspberry PI is a small ARM computer (hardware specifications in wiki, outline and FAQs). Of course, you can build a cluster with it!

As 2π is proposed to be named τ we’ve chosen the name “Raspberry Tau” for this proof-of-concept.

We’ve connected two Raspberry Pis via their on-board ethernet interfaces (via a switch, so we can simply SSH into them), booted via 2GB SD-cards with a Raspbian image on them. After upgrading to a kernel that has kernel-headers available we built DRBD modules, and voilá! A Raspberry Tau cluster is born.

We’re replicating the data on the USB-Sticks; their performance nicely matches the available network. Here’s /proc/drbd (shortened and line-wrapped for readability):

[email protected]:~# cat /proc/version 
Linux version 3.2.0-3-rpi (Debian 3.2.21-1+rpi1) 
  ([email protected]) (… Debian 4.6.3-1.1+rpi2)…)
[email protected]:~# cat /proc/drbd 
version: 8.4.2 (api:1/proto:86-101)
GIT-hash: 7ad5f850d711223713d6dcadc3dd48860321070c build by
    [email protected], 2012-09-18 12:58:08
 0: cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r-----
    ns:805304 nr:0 dw:348628 dr:818596 al:127 bm:70 lo:0 pe:0
      ua:0 ap:0 ep:1 wo:d oos:0

As Raspbian is Debian-based, there are Pacemaker (and Heartbeat resp. Corosync) packages available … so a cheap, low-power, High-Availability cluster is easily built.

Disclaimer: for a real HA-cluster you’d need a few more things.

  • a STONITH device (if power is supplied via a Linux-PC, you could turn off the USB port by software), and
  • redundant network connectivity (USB ethernet adapter).

Of course, if you’re just clustering your media library these things might not be mandatory.

Packages are available for everyone – just drop an email to [email protected], and we will be happy to provide them. Please note that the upstream kernels already include the DRBD kernel module, so there’s no need to compile your own – unless you want to test the cutting edge; in that case, good luck 😉

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