Pegasus IV Cluster
How to build a diskless Linux cluster?
Reasons why you would want your compute nodes to be diskless:
money saved on hard disks
Of course, there are some disadvantages, too:
root file system occupies part of the compute nodes' RAM and reduces the amount of RAM available for applications
Root-NFS versus Root-RAM file system
Where to put the root file systems of the compute nodes, in the compute node RAM or on the server, mounted via NFS? We strongly prefer the Root-RAM setup mainly for three reasons:generates less network traffic
better scalability to many compute nodes; in the (naive) Root-NFS approach one needs a separate file system on the server
for each of the nodes while the same image can be used to populate all of the RAM file systems
compute nodes are not just diskless, they are stateless; after a reboot they are guaranteed to have the same well-defined
configuration which does not wander in time
Pegasus Cluster concept
cluster server runs standard installation of Scientific Linux 7.1
+ some extra packages to facilitate the cluster operation
The Pegasus Cluster concept is flexible; it supports heterogeneous clusters as long as all compute nodes can run 64-bit x86 Scientific Linux. Differences in node hardware can be dealt with by distributing different Linux kernels and root file systems to different types of nodes (see Node Installation).
We have used the Pegasus Cluster concept for clusters with almost 200 compute nodes, without notable strain on the server. We thus believe it can be scaled up beyond 200 nodes. Large clusters of several thousand nodes probably require changes to the node provisioning mechanism to avoid overloading the server.