2.5 CD-ROM-Based Clusters

If you just want to learn about clusters, only need a cluster occasionally, or can't permanently install a cluster, you might consider one of the CD-ROM-based clusters. With these, you create a set of bootable CD-ROMs, sometimes called "live filesystem" CDs. When you need the cluster, you reboot your available systems using the CD-ROMs, do a few configuration tasks, and start using your cluster. The cluster software is all available from the CD-ROM and the computers' hard disks are unchanged. When you are done, you simply remove the CD-ROM and reboot the system to return to the operating system installed on the hard disk. Your cluster persists until you reboot.

Clearly, this is not an approach to use for a high-availability or mission-critical cluster, but it is a way to get started and learn about clusters. It is a viable way to create a cluster for short-term use. For example, if a computer lab is otherwise idle over the weekend, you could do some serious calculations using this approach.

There are some significant difficulties with this approach, most notably problems with storage. It is possible to work around this problem by using a hybrid approach-setting up a dedicated system for storage and using the CD-ROM-based systems as compute-only nodes.

Several CD-ROM-based systems are available. You might look at ClusterKnoppix, http://bofh.be/clusterknoppix/, or Bootable Cluster CD (BCCD), http://bccd.cs.uni.edu/. The next subsection, a very brief description of BCCD, should give you the basic idea of how these systems work.

2.5.1 BCCD

BCCD was developed by Paul Gray as an educational tool. If you want to play around with a small cluster, BCCD is a very straightforward way to get started. On an occasional basis, it is a viable alternative. What follows is a general overview of running BCCD for the first time.

The first step is to visit the BCCD download site, download an ISO image for a CD-ROM, and use it to burn a CD-ROM for each system. (Creating CD-ROMs from ISO images is briefly discussed in Chapter 4.) Next, boot each machine in your cluster from the CD-ROM. You'll need to answer a few questions as the system boots. First, you'll enter a password for the default user, bccd. Next, you'll answer some questions about your network. The system should autodetect your network card. Then it will prompt you for the appropriate driver. If you know the driver, select it from the list BCCD displays. Otherwise, select "auto" from the menu to have the system load drivers until a match is found. If you have a DHCP and DNS server available on your network, this will go much faster. Otherwise, you'll need to enter the usual network configuration information-IP address, netmask, gateway, etc.

Once the system boots, log in to complete the configuration process. When prompted, start the BCCD heartbeat process. Next, run the utilities bccd-allowall and bccd-snarfhosts. The first of these collects hosts' keys used by SSH and the second creates the machines file used by MPI. You are now ready to use the system.

Admittedly, this is a pretty brief description, but it should give you some idea as to what's involved in using BCCD. The boot process is described in greater detail at the project's web site. To perform this on a regular basis with a number of machines would be an annoying process. But for a few machines on an occasional basis, it is very straightforward.