Building a Blender cluster with Xserves I have three 2009-era Xserves that I need to assemble into a compute cluster/render farm for Blender. The Xserves do not have any operating system on them. What OS should I use for best results with Blender? Do I need OS X Server or can I get away with the client version or even Linux? 

I've never assembled a cluster, nor do I have any server experience to draw from.

Hardware:


3 Xserves (specifically, the Xserve3,1 model)
5 CPU (20 cores total): 2.26 GHz quad core Intel Xeon 5500 series “Nehalem” processors (two CPUs each on two Xserves)
6 GB RAM on one of the dual CPU servers, 3 GB RAM on the remaining two Xserve
no Fiber Cards or Raid cards to complicate driver selection
no PCI cards
each server has a built-in NVIDIA GeForce GT 120 with a mini DisplayPort output and 256 MB of video RAM
If you read the system requirements for Blender, it's pretty wide open. You could run Windows XP or newer, Mac OS X version 10.5 or newer and of course you could run Linux or perhaps even FreeBSD.

So, it's really your call which OS is easiest for you to maintain. For free (or pretty near to free) you could get a used copy of Snow Leopard or Leopard for probably $15 or even call Apple or go to an Apple store with the serial numbers and see what OS was shipped with those Macs. You might have to pay a shipping fee, but they will mail you replacement DVD for the OS.

You certainly don't need server for this software (reading the wiki linked in the question - it's clear the software handles all the clustering needed as long as all the devices are on the same network and can talk).

It depends on how expensive your time and what OS you like best to run and install. Mac is clearly the easiest since all the drivers you need will be vetted and ready to go. Once you have the software installed and running - should you find in discussions with the blender community that another OS handles a specific task better - you could always jump ship to that OS later.

Looking over the Xserve specs, you might not be able to run 10.5 but unless you have a big reason to not upgrade - I'd run the latest client version of Mac OS that will run on that hardware.