Separate Functions

If nodes act as either compute servers or as I/O servers, but not as both, all parallel I/O operations will generate network traffic and the node's network interface will determine the limit of the performance of a parallel file system. In such cases, the total number of processing nodes being used to run the processes of a parallel job will set an upper limit on the aggregate throughput available. The absolute limit will be set by the bandwidth limitations of the network interconnect itself.

For example, if a sixteen-process job is scheduled on four SMP nodes, then the limiting factor will be the four network adaptors that the SMPs will use for communicating with the remote storage objects of the parallel file system.

In such cases, the best rule of thumb is to match (as closely as possible) the number of compute nodes to the number of I/O nodes so that consumer bandwidth is roughly matched to producer bandwidth within the limitations of the cluster's network bandwidth.