3.1. Starting and Stopping Tracing on Compute Nodes

After you install the App Trace Roll on your frontend and compute nodes, the compute nodes will be running a SystemTap script that is ready to collect data about each read and write I/O for an application (or set of applications).

To start colleting data about the program iozone, execute:

# cluster-fork 'echo "iozone" > /proc/sys/debug/traced-apps'

After the above command completes, when you run iozone on any compute node, the SystemTap script will record its I/O activity to a log file.

To trace multiple programs, just separate them by a comma. For example, to trace both iozone and dd:

# cluster-fork 'echo "iozone,dd" > /proc/sys/debug/traced-apps'

To stop tracing, execute:

# cluster-fork 'echo "" > /proc/sys/debug/traced-apps'