IRIDIA cluster architecture

The configuration of our cluster has two ``heads, that is, two servers share the work and, if possible, replicate the services in order to be more failsafe. The servers are polyphemus and majorana.

Majorana provides the following services:

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

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.