One root filesystem

oneSIS was developed as a system that could essentially be a cluster `building block' of sorts. It does this by adhering to one simple constraint: the root filesystem of every node should always be exactly identical. Every node is an exact mirror of the root image. This property has many desirable benefits in terms of system manageability and scalability.

The root filesystem of every node is bit-for-bit identical whether it resides on a local disk or is mounted via NFS. Functional groups of nodes or individual nodes can be configured to behave differently by using `linkbacks' to configure themselves at boot-time with symbolic links going to and from a small RAM disk.

For large clusters, diskful mirrors of the image can each NFS export their root filesystem to a reasonable number of `diskless' nodes so that services such as DHCP, TFTP, and NFS of the root image can be distributed efficiently to any scale necessary using commodity hardware.