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## 8. Clusters

In this section I will briefly touch on the ways machines can be connected together but this is so big a topic it could be a separate HOWTO in its own right, hint, hint. Also, strictly speaking, this section lies outside the scope of this HOWTO, so if you feel like getting fame etc. you could contact me and take over this part and turn it into a new document.

These days computers gets outdated at an incredible rate. There is however no reason why old hardware could not be put to good use with Linux. Using an old and otherwise outdated computer as a network server can be both useful in its own right as well as a valuable educational exercise. Such a local networked cluster of computers can take on many forms but to remain within the charter of this HOWTO I will limit myself to the disk strategies. Nevertheless I would hope someone else could take on this topic and turn it into a document on its own.

This is an exciting area of activity today, and many forms of clustering is available today, ranging from automatic workload balancing over local network to more exotic hardware such as Scalable Coherent Interface (SCI) which gives a tight integration of machines, effectively turning them into a single machine. Various kinds of clustering has been available for larger machines for some time and the VAXcluster is perhaps a well known example of this. Clustering is done usually in order to share resources such as disk drives, printers and terminals etc, but also processing resources equally transparently between the computational nodes.

There is no universal definition of clustering, in here it is taken to mean a network of machines that combine their resources to serve users. Admittedly this is a rather loose definition but this will change later.

These days also Linux offers some clustering features but for a starter I will just describe a simple local network. It is a good way of putting old and otherwise unusable hardware to good use, as long as they can run Linux or something similar.

One of the best ways of using an old machine is as a network server in which case the effective speed is more likely to be limited by network bandwidth rather than pure computational performance. For home use you can move the following functionality off to an older machine used as a server:

- news
- mail
- web proxy
- printer server
- modem server (PPP, SLIP, FAX, Voice mail)

You can also `NFS mount` drives from the server onto your workstation thereby reducing drive space requirements. Still read the FSSTND to see what directories should not be exported. The best candidates for exporting to all machines are `/usr` and `/var/spool` and possibly `/usr/local` but probably not `/var/spool/lpd`.

Most of the time even slow disks will deliver sufficient performance. On the other hand, if you do processing directly on the disks on the server or have very fast networking, you might want to rethink your strategy and use faster drives. Searching features on a web server or news database searches are two examples of this.

Such a network can be an excellent way of learning system administration and building up your own toaster network, as it often is called. You can get more information on this in other HOWTOs but there are two important things you should keep in mind:

- Do not pull IP numbers out of thin air. Configure your inside net using IP numbers reserved for private use, and use your network server as a router that handles this IP masquerading.
- Remember that if you additionally configure the router as a firewall you might not be able to get to your own data from the outside, depending on the firewall configuration.

The Nyx network provides an example of a cluster in the sense defined here. It consists of the following machines:

nyx

  is one of the two user login machines and also provides some of the networking services.

nox

  (aka nyx10) is the main user login machine and is also the mail server.

noc

  is a dedicated news server. The news spool is made accessible through NFS mounting to nyx and nox.

arachne

  (aka www) is the web server. Web pages are written by NFS mounting onto nox.

There are also some more advanced clustering projects going, notably

- The Beowulf Project
- The Genoa Active Message Machine (GAMMA)

High-tech clustering requires high-tech interconnect, and SCI is one of them. To find out more you can either look up the home page of Dolphin Interconnect Solutions which is one of the main actors in this field, or you can have a look at scizzl.

Centralised mail servers using IMAP are becoming more and more popular as disks become large enough to keep all mail stored indefinitely and also cheap enough to make it a feasible option. Unfortunately it has become clear that `NFS` mounting the mail archives from another machine can cause corruption of the IMAP database as the server software does not handle NFS timeouts too well, and NFS timeouts are a rather common occurrence. Keep therefore the mail archive local to the IMAP server.

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