IRIDIA cluster architecture

Introduction

The cluster was built in 2002 and has been extended and modified since. Currently it consists of two servers and a number of nodes with disks and a number with out disks. The disk-less nodes are the normal PC looking boxes on the shelves in the server room, while the nodes with disks are the ones in the rack.

Currently, servers as well as nodes run a 32-bit Debian GNU/Linux, however the nodes in the rack are dual Opterons so that might change at some point in the future.

majorana is the main server and provides the following services:

• NTP
• NIS
• Sun Grid Engine
• Vortex License
• DHCP for the computer on the shelves
• TFTP for diskless booting

Normally, users will log on to marjoana and submit jobs using the Sun Grid Engine.

Physical setup

The figure shows the cluster architecture, notice that each of the two servers, majorana and polyphemus, double as computing nodes in each of their part of the cluster. That is majorana is also r01 while polyphemus is also p01.

The boot process

The task of maintenance of the cluster is much easier if one knows the boot sequences of the clients. In fact, if the sequences are known, it is easier to find the source of errors and which files should be modified.

There are two kind of clients in the cluster, disk-less (the shelf-cluster) and with-disk (rack cluster). The boot sequence is obviously different for both, and therefore are explained separately.

Here is, step by step, what happens when a disk-less computer boots. majorana is in charge of handling this process. All configuration files are on it.

1. The clients is switched on. Its ethernet card sends a DHCP request in broadcast on the network. The program in the card's firmware that deals with this is called PXE (PXE is actually one of the standards that can be used to boot, and is developed mainly by Intel. 3com cards, for instance, require different procedures that the one described here in order to receive a kernel to boot.)

   The BIOS of the client must be configured in order to enable PXE. Moreover, the card should be set as first booting device. 

2. majorana receives the requests and start a DHCP dialog with the client. During the dialog, the server tells the client which is its IP address, its name, the default gateways, name and time servers. Most important, it tells to which computer the client should address to receive the kernel (majorana again), which is the file to request (pxelinux.0) and where to find the root image to mount (on majorana).

   On majorana, /etc/dhcp.conf or /etc/dhcp3/dhcp.conf. pxelinux.0 is part of the syslinux package.

Redundancy and replication

Notice that none of the things mentioned below has actually been installed - this is merely a wish list and ideas

Two servers provide two access points to the cluster. The services required by the nodes are splitted on the servers in order to reduce the workload and to improve robustness to failures. Here there is a description of which services can be duplicated and what needs to be done in case of crashes.

• The NIS protocol already includes the presence of more that one server, of which only one is the master server. The others are slave servers that are a copy of the master and that work only when the master is unreachable.
• NTP is used to keep the clocks of the cluster syncronized. The clients can access only one server (to check!), although there might be more in the network. A failure in the NTP server is not considered critical, because it will take days before the clocks of the clients differ unreasonably. Therefore, only one server is enough
• The Vortex License server cannot be copied, and it is already configured so that it can run only on polyphemus. If polyphemus crashes, it is still possible to start it on majorana by changing the MAC address of the latter to 00:0C:6E:02:41:C3 (polyphemus's MAC address). majorana can copy the file needed to run the server on a daily basis.
• Sun Grid Engine (SGE) can be run only on one computer (polyphemus). All the nodes access its data via NFS. majorana can copy SGE directory daily, but if polyphemus crashes, all the nodes must be instructed to mount the new directory on majorana.
• /home directories. There can be only one NFS server in the network. majorana was chosen because it react faster: it has 2 CPUs, and when one is busy writing, the other can still process other incoming requests. Both majorana and polyphemus use RAID architecture to prevent data loss. The only problem is if majorana is not reachable any more. In this case, each process on the nodes that tries to access \texttt{/home} will be stopped till majorana comes up again.
• The root directories of the diskless nodes are on polyphemus. If polyphemus is not reachable, these nodes will be blocked waiting for polyphemus to come up again. majorana could keep a backup the these directories, but if the administrator wants to mount the backup directories on majorana, the nodes must be manually rebooted because they are note reachable via SSH. The DHCP configuration must also be changed to give the new mount point of the backup directories to the nodes.
• In this solutions, two different DHCP servers deals with the two groups because of the different way of maintaining and updating the nodes. Anyway, only one server could do the same job. In case of failure of one server, the other can restart the DHCP server with a new configuration to deal with the whole cluster.