Copying Data to the Compute Nodes

There are several ways to get data from the master node to the compute nodes. This section describes using NFS to share data, using the Scyld ClusterWare command bpcp to copy data, and using programatic methods for data transfer.

Sharing Data via NFS

The easiest way to transfer data to the compute nodes is via NFS. All files in your /home directory are shared by default to all compute nodes via NFS. Opening an NFS-shared file on a compute node will, in fact, open the file on the master node; no actual copying takes place.

Copying Data via bpcp

To copy a file, rather than changing the original across the network, you can use the bpcp command. This works much like the standard Unix file-copying command cp, in that you pass it a file to copy as one argument and the destination as the next argument. Like the Unix scp, the file paths may be qualified by a computer host hame.

With bpcp, you can indicate the node number for the source file, destination file, or both. To do this, prepend the node number with a colon before the file name, to specify that the file is on that node or should be copied to that node. For example, to copy the file /tmp/foo to the same location on node 1, you would use the following command:

[user@cluster username]$ bpcp /tmp/foo 1:/tmp/foo

Programmatic Data Transfer

The third method for transferring data is to do it programatically. This is a bit more complex than the methods described in the previous section, and will only be described here only conceptually.

If you are using an MPI job, you can have your Rank 0 process on the master node read in the data, then use MPI's message passing capabilities to send the data over to a compute node.

If you are writing a program that uses BProc functions directly, you can have the process first read the data while it is on the master node. When the process is moved over to the compute node, it should still be able to access the data read in while on the master node.

Data Tranfer by Migration

Another programmatic method for file transfer is to read a file into memory prior to calling BProc to migrate the process to another node. This technique is especially useful for parameter and configuration files, or files containing the intermediate state of a computation. See the Reference Guide for a description of the BProc system calls.