Storage Area Network: Architecture

Host Bus Adapters

A host bus adapter (HBA) is a PCI adapter card that resides in the server whose only function is to manage and transfer information between the host server and a Fibre Channel device. The HBA performs many low-level interface functions automatically with minimal processor involvement.

An HBA can utilize either copper or optical cable. When two HBAs are present in a server, redundancy is established, providing two paths for data transmission.

Execution Throttle

Some HBAs feature a setting called the execution throttle. This setting specifies the maximum number of commands executing on any one port. When a port's execution throttle is reached, no new commands are executed until the current command finishes executing.

Generally, if three or more servers exist in a SAN configuration, the default execution throttle will need to be changed. If all servers carry the same I/O load, calculate the value by dividing 240 by the number of servers in the SAN. Then, set each HBA in the SAN to the calculated value. For example, in a four-server configuration, divide 240 by 4 to arrive at 60. The execution throttle value for each HBA is 60.

NOTE: The sum of execution throttle values should never exceed the maximum queue depth of 240.