The Host sFlow agent exports physical and virtual server performance
metrics using the sFlow protocol. The agent provides scalable,
multi-vendor, multi-OS performance monitoring with minimal impact on
the systems being monitored.

To start hsflowd at boot, add something like this to your
/etc/rc.d/rc.local:

if [ -x /etc/rc.d/rc.hsflowd ]; then
  /etc/rc.d/rc.hsflowd start
fi