Performance characterization of large and long fibre channel arbitrated loops

The bandwidth performance of a Fibre Channel Arbitrated Loop (FCAL) is roughly defined to be 100 MegaBytes (100 bytes) per second. Furthermore, FCAL is capable of a theoretical peak of 40,000 I/O operations (transactions) per second. These performance levels, however, are largely not realized by the applications that use Fibre Channel as an interface to disk subsystems. The bandwidth and transaction performance of an Arbitrated Loop is sensitive to both the number of devices on the loop as well as the physical length of the loop. This study focuses on the effects of these two factors on the observed performance of Fibre Channel Arbitrated Loop as the number of nodes is scaled from 2 to 97 devices and as the physical length of the loop is scaled from 50 meters to several kilometers in length. To summarize, this study shows that the performance decreases significantly for very long loops and explains how this can be partially avoided. Also, the loop propagation delay on loops with many devices has only a moderate affect on performance. Finally, the effects of length tend to dominate the effects of population for very long, highly populated loops.