2.4.2. Monitoring Bandwidth

Monitoring bandwidth is more difficult than the other resources described here. The reason for this is due to the fact that performance statistics tend to be device-based, while most of the places where bandwidth is important tend to be the buses that connect devices. In those instances where more than one device shares a common bus, you might see reasonable statistics for each device, but the aggregate load those devices place on the bus would be much greater.
Another challenge to monitoring bandwidth is that there can be circumstances where statistics for the devices themselves may not be available. This is particularly true for system expansion buses and datapaths[5]. However, even though 100% accurate bandwidth-related statistics may not always be available, there is often enough information to make some level of analysis possible, particularly when related statistics are taken into account.
Some of the more common bandwidth-related statistics are:
Bytes received/sent
Network interface statistics provide an indication of the bandwidth utilization of one of the more visible buses -- the network.
Interface counts and rates
These network-related statistics can give indications of excessive collisions, transmit and receive errors, and more. Through the use of these statistics (particularly if the statistics are available for more than one system on your network), it is possible to perform a modicum of network troubleshooting even before the more common network diagnostic tools are used.
Transfers per Second
Normally collected for block I/O devices, such as disk and high-performance tape drives, this statistic is a good way of determining whether a particular device's bandwidth limit is being reached. Due to their electromechanical nature, disk and tape drives can only perform so many I/O operations every second; their performance degrades rapidly as this limit is reached.

[5] More information on buses, datapaths, and bandwidth is available in Chapter 3, Bandwidth and Processing Power.