Today, LAN managers have to choose between a wide variety of array solutions. In addition to RAID feature comparisons, the LAN manager must also choose between a host-based array RAID controller or a SCSI-to-SCSI based array solution, also sometimes referred to as a 'bridge controller'.
Host-based array controllers are designed to be installed 'inside' a host server, directly connected to an I/O bus such as EISA or PCI. Conversely, SCSI-to-SCSI array controllers are designed to be installed 'outside' of the host server, typically inside an external array enclosure, and directly connected to an additional SCSI host adapter which is located inside the server. Besides the fact that SCSI-to-SCSI array solutions require two adapters (the bridge controller itself plus an additional host adapter), this design also is burdened by at least two other design flaws — poor array management and encumbered performance.
High on a LAN manager's list of requirements for array solutions is array management and monitoring from any client within the LAN. Host-based solutions provide this functionality through network-ready storage management software. Since a host-based array controller is installed in the server, the storage management software is able to directly communicate with the array controller to effectively manage and monitor the array from any client within the LAN.
SCSI-to-SCSI solutions require an additional SCSI host adapter, which could be from any manufacturer, to communicate with the host. This means that direct communication and array management from any client within the LAN is not possible. To facilitate array management and monitoring, SCSI-to-SCSI solutions offer an LCD panel on an external array enclosure, or a character-based, serial connection on the SCSI-to-SCSI controller itself. This scheme not only limits array management and monitoring functionality, but also forces the LAN manager to be physically present to perform any array management.
Also high on a LAN manager's list of requirements for array solutions is performance! When looking at SCSI-to-SCSI solutions, it is important to realize that although most offer multiple SCSI channels for array configurations, they are unfortunately limited to a just a single SCSI channel back to the host server. This introduces a significant performance bottleneck. For example, how does a SCSI-to-SCSI controller which offers four Wide SCSI channels (80MB/second) squeeze that data into a single Wide SCSI (20MB/second) channel back to the server without negatively impacting performance?
Host-based array solutions do not exhibit this performance limitation since they are directly connected to the host I/O bus which is capable of transferring data at up to 132MB/second on a PCI bus. Therefore, a host-based array controller with multiple Wide or Ultra SCSI channels is not performance bottlenecked at the host.
After considering the differences between the two approaches and the impact each has on system performance, it is apparent that SCSI-to-SCSI may not be the best choice for high-volume servers.